LIBRARY 

OF    THE 

UNIVERSITY  OF  CALIFORNIA. 
Class 


THE  STUDY  OF  THE  ATOM;    OR,  THE 
FOUNDATIONS  OF  CHEMISTRY. 


THE  STUDY  OF  THE  ATOM  ; 


OR, 


THE  FOUNDATIONS  OF  CHEMISTRY. 


BY 


F.  P.  VENABLE,  PH.D.,  D.Sc.,  LL.D., 


UNIVERSITY  OF  NORTH  CAROLINA, 


BASTON,  PA.  : 

THE  CHEMICAL  PUBLISHING  COMPANY. 
1904. 


COPYRIGHT,  1904,  BY  THE  CHEMICAL  PUBLISHING   Co. 


TABLE  OF  CONTENTS. 


CHAPTER  I. 

ANCIENT  VIEWS  AS  TO  THE  NATURE  OF  MATTER. 

The  Constitution  of  Matter,  p.  5.  Theory  and  Empiricism,  p.  5. 
Purpose  of  the  Book,  p.  6.  The  Two  Theories,  p.  6.  Chinese 
Theories,  p.  6.  Hindoo  Theories,  p.  7.  Atomic  Theory  of 
Kanada,  p.  8.  Mathematical  Derivation  of  the  Theories,  p.  9. 
Greek  Theories,  p.  10.  Ionic  School,  p.  u.  Thales  of  Miletus, 
600  B.  C.,  p.  12.  Anaximander,  546  B.  C.,  p.  12.  Anaximenes, 
p.  12.  Pythagorean  School,  p.  13.  Anaxagoras,  500  B.  C.,  p.  14. 
Heraclitus,  500  B.  C.,  p.  16.  Empedocles,  500  B.  C.,  p.  16. 
Leucippus,  500  B.  C.,  p.  18.  Eleatic  School,  p.  19.  Democritus, 
460  B.  C.,  p.  19.  Plato,  427  B.  C.,  p.  22.  Heracleides,  p.  23. 
Aristotle,  384  B.  C.,  p.  23.  Epicurus,  342  B.  C.,  p.  27.  Sum- 
mary, p.  29.  The  Greeks  as  Observers,  p.  30.  Modern  Methods, 
p.  32.  Failure  of  Greeks,  p.  32.  Arguments  of  Zeno,  p.  33. 
Contributions  of  the  Ancients,  p.  34. 

CHAPTER  II. 

FROM  THE  GREEK  PHILOSOPHERS  TO  DAI/TON. 

Development  of  Experimental  Science,  p.  39.  Eclipse  of  Knowl- 
edge, p.  42.  The  Alchemists,  p.  43.  Opposition  of  the  Church, 
p.  43.  Kinds  of  Atoms,  p.  47.  General  Theories,  p.  48.  In- 
fluence of  Aristotle,  p.  49.  Arabian  Theories,  p.  50.  Mathe- 
matical Views,  p.  51.  Condition  of  Elements  in  Compounds,  p. 
51.  Van  Helmont  and  the  Corpuscular  Theory,  p.  52.  Giordano 
Bruno,  1548-1600,  p.  53.  Lubin,  1565-1631,  p.  54.  Francis 
Bacon,  1561-1626,  p.  54.  Daniel  Sennert,  1572-1637,  p.  55. 
Other  Theorists,  p.  57.  Galileo  and  the  Italian  School,  p.  58. 
Descartes  and  the  Corpuscular  Theory,  p.  59.  The  Vacuum  of 
Torricelli,  p.  61.  Thomas  Hobbes,  1588-1679,  p.  62.  Robert 
Boyle,  1627-1691,  p.  64.  Vibration  Theory  of  Hooke,  p.  66. 
Huygens,  1629-1695,  p.  67.  Attacks  of  the  Church,  p.  69.  Leib- 
nitz, 1646-1716,  p.  69.  Newton,  1643-1727,  p.  70.  Boscovich, 
p.  74.  Neglect  of  Hypotheses,  p.  75. 


iv  TABI,E  OF  CONTENTS. 

CHAPTER  III. 

THE  ATOMIC  THEORY  OF  CHEMISTRY. 

Justice  of  Dalton's  Claim,  p.  80.  Fundamental  Laws,  p.  80. 
Lavoisier  and  the  New  Chemistry,  p.  81.  Law  of  Interpropor- 
tionality,  83.  Wenzel,  p.  83.  Richter,  p.  84.  Law  of  Multiple 
Proportion,  p.  87.  W.  Higgins,  p.  89.  Dalton's  Theory,  p.  90. 
Thomson's  Account,  p.  91.  Dalton's  Lecture  Notes,  p.  92.  De- 
ductions from  Other  Papers,  p.  96.  First  Publication  of  the 
Theory  by  Dalton,  p.  99.  First  Table  of  Atomic  Weights,  p. 
101.  Inception  of  the  Theory,  p.  103.  Conclusions  of  Roscoe 
and  Harden,  p.  104.  Details  of  Dalton's  Theory,  p.  105.  Recep- 
tion of  the  Theory,  p.  106.  Extension  of  the  Theory  by  Ber- 
zelius,  p.  108.  Dalton's  Rules  for  Determining  the  Atomic 
Weights,  p.  109.  Inconsistencies  Bring  Disfavor,  p.  no.  Terms 
Substituted  for  Atoms,  p.  in.  Combining  Volumes,  p.  112. 
Confusion  and  Division,  p.  113. 

CHAPTER  IV. 

THE  RELATIVE  WEIGHTS   OF  THE  ATOMS. 

Gaseous  Molecules,  Law  of  Pressures,  p.  117.  Law  of  Tempera- 
tures, p.  118.  Law  of  Volumes,  p.  119.  Opposition  of  Dalton, 
p.  120.  Law  of  Densities,  p.  121.  Theory  of  Avogadro,  p.  122. 
Distinction  between  Atoms  and  Molecules,  p.  123.  Application 
of  the  Distinction,  p.  124.  Confirmation  of  the  Theory,  p.  126. 
Nascent  State,  p.  126.  Ozone,  p.  127.  Intramolecular  Work,  p. 
129.  Practical  Use  of  the  Theory,  p.  130.  Results  as  to  Gaseous 
Molecules,  p.  131.  Supposed  Exceptions  to  the  Theory,  p.  132. 
Law  of  Specific  Heats,  p.  134.  Difficulties  in  the  Way  of  Ac- 
ceptance, p.  136.  Application  to  Compounds,  p.  136.  Experi- 
mental Difficulties,  p.  138.  Apparent  Exceptions,  p.  140.  Fail- 
ures of  the  Law,  p.  141.  Hypothesis  of  Kopp,  p.  142.  Law  of 
Isomorphism,  p.  143.  Early  Theories,  p.  144.  Theory  of 
Mitscherlich,  p.  145.  Conclusions  Drawn  Uncertain,  p.  147. 
Complexity  of  the  Problem,  p.  147.  Electrochemical  Equiva- 
lents, p.  148.  Freezing-Points  of  Solutions,  p.  150.  Vapor- 
Pressures  and  Boiling-Points,  p.  150.  Atomic  Theory  in  Doubt, 
p.  151.  Atom  and  Molecule,  p.  151.  Cannizzaro's  Views,  p. 
154.  Congress  of  Carlsruhe,  p.  154.  Later  Progress,  p.  155. 
Standard  for  the  Atomic  Weights,  p.  156.  The  Dalton  Standard, 
p.  156.  Berzelian  Standard,  p.  157.  The  Value  Assigned  the 
Standard,  p.  158. 


TABLE  OF   CONTENTS.  V 

CHAPTER  V. 

THE  PERIODIC  OR  NATURAL  SYSTEM. 

Prout's  Hypothesis,  p.  164.  Berzelius'  Antagonism,  p.  165.  Fate 
of  the  Hypothesis,  p.  165.  Numerical  Regularities,  p.  166. 
Dobereiner's  Triads,  p.  167.  Gladstone's  Ascending  Series,  p. 
1 68.  Homologous  Series,  p.  169.  Telluric  Screw,  p.  169.  Law 
of  Octaves,  p.  170.  Hinrichs  on  the  Properties,  p.  172.  Meyer's 
Table,  p.  172.  Table  of  Mendele'eff,  p.  173.  Basis  of  a  Natural 
System,  p.  177.  Periodicity  of  Properties,  p.  178.  Graphic 
Representation,  p.  178.  Analogies  of  the  Elements,  p.  183. 
Periodicity  of  the  Elements,  p.  184.  Difficulties  of  the  System, 
p.  185.  Position  of  Hydrogen,  p.  186.  Genesis  of  the  Elements, 
p.  187.  Hypothesis  of  Crookes,  p.  188.  Evolution  Theories,  p. 
190.  Are  the  Elements  Composite  or  Simple  ?  p.  190.  Evi- 
dence as  to  Complexity,  p.  191. 

CHAPTER  VI. 

AFFINITY,   THE  ATOMIC   BINDING  FORCE. 

Early  Views  of  Affinity,  p.  195.  Strength  of  Affinity,  p.  197. 
Definition  of  Affinity,  p.  199.  Is  Affinity  a  Distinct  Force  ?  p. 
199.  Explanation  Offered  by  Berzelius,  p.  201.  Measurement 
of  Affinity,  p.  202.  Disturbing  Influences,  p.  203.  Heat  of 
Chemical  Reactions,  p.  204.  Deductions  from  Thermochemistry, 
p.  205.  Molecular  Affinity,  p.  206.  Heat  of  Neutralization,  p. 
206.  Mass  Action,  p.  208.  Support  for  Berthollet's  Views,  p- 
209.  Influence  of  Heat  upon  Affinity,  p.  213.  Kinetic  Theory, 
p.  213.  Theory  of  Ions,  p.  214.  Guldberg  and  Waage,  p.  215. 
Velocity  of  Chemical  Change,  p.  217.  Other  Methods,  p.  217. 
Conclusions,  p.  218.  Molecular  Attraction,  p.  219. 

CHAPTER  VII. 
VALENCE. 

Definition  of  Valence,  p.  223.  Evolution  of  the  Idea,  p.  225. 
Polybasic  Acids,  p.  225.  The  Work  of  Frankland,  p.  226.  A 
Relative  Property,  p.  227.  Valence  Variable,  p.  228.  Molecular 
Combination,  p.  230.  Objections  to  the  Hypothesis,  p.  230. 
Hypothesis  of  Werner,  p.  231.  Nitrogen  Both  Trivalent  and 
Quinquivalent,  p.  232.  Saturated  and  Unsaturated,  p.  233. 
Bonds  or  I/inks,  p.  234.  Equality  of  Bonds,  p.  235.  Self-Satu- 
ration, p.  236.  Change  in  Valence  Radical,  p.  236.  Causes  of 


VI  TABLE  OF  CONTENTS. 

Change  in  Valence,  p.  237.  Changes  Caused  by  Light,  p.  237. 
Changes  Caused  by  Heat,  p.  238.  Changes  Caused  by  Elec- 
tricity, p.  239.  Changes  Caused  by  Chemical  Action,  p.  239. 
Hypothesis  of  van't  Hoff,  p.  240.  Ostwald's  Views,  p.  241. 
Lossen  on  Valence,  p.  242.  Wislicenus  on  Valence,  p.  243. 
Hypothesis  of  Victor  Meyer  and  Riecke,  p.  243.  Hypothesis  of 
Knorr,  p.  244.  Hypothesis  of  Flawitzky,  p.  244.  Kekule's 
Views,  245.  Other  Views,  p.  245.  Hypothesis  of  Richards,  p. 
246.  Hypothesis  of  Venable,  p.  246. 

CHAPTER  VIII. 

MOLECULES  AND  THE  CONSTITUTION  OF  MATTER. 

The  Influence  of  Atom  upon  Atom,  p.  252.  Nascent  State,  p.  253. 
Allotropism,  p.  254.  Further  Cases  of  Atom  Influencing  Atom, 
p.  254.  Isomerism,  p.  255.  Influence  of  Position,  p.  256.  Effect 
of  Position  in  Space,  p.  259.  An  Explanation  in  the  Kinetic 
Theory,  p.  260.  Properties  of  Molecules,  p.  261.  Molecular 
Motion,  p.  261.  Properties  of  the  Molecule,  p.  262.  Experi- 
mental Investigations,  p.  263.  Divisibility  of  Matter,  p.  265. 
Rankine's  Hypothesis,  p.  265.  Vortex  Atoms,  p.  266.  Thom- 
son's Theory,  p.  266.  Properties  of  the  Vortex,  p.  268.  Conse- 
quences of  the  Theory,  p.  268.  The  Vortex  Theory  Applied  to 
Valence,  p.  269.  The  Vortex  Theory  and  Affinity,  p.  269. 
Electron  Hypothesis,  p.  270.  Crookes'  Summary,  p.  272. 
Lodge  on  Modern  Views  of  Matter,  p.  277.  Rutherford's  Hy- 
pothesis, p.  278.  Clarke  on  the  Atomic  Theory,  p.  279.  The 
Ether  of  Mendele"eff,  p.  280. 

Index 283 


INTRODUCTION. 

The  purpose  of  this  work  is  to  trace  the  atomic  theory 
of  chemistry  from  its  earliest  conception  to  the  present 
day.  This  forms  the  foundation  of  all  chemical  theory 
and  has  been  offered  as  the  best  explanation  of  the  con- 
stitution of  matter  and  of  the  universe.  This  theory  has 
had  a  longer  life  than  any  other  philosophical  or  scien- 
tific conception,  and  has  to-day  more  nearly  its  ancient 
form.  It  has  lived  through  bitter  attack,  dialectic  strife, 
and  even  persecution,  and  can  number  its  martyrs.  It 
has  called  to  its  service  the  master  minds  of  the  world  and 
the  greatest  ingenuity  in  experiment  and  in  logic.  It  is 
not  to  be  presumed  that  such  a  conception  can  be  dis- 
missed in  a  few  slighting  sentences  or  overturned  by  one 
or  two  crude  hypotheses. 

It  is  no  part  of  the  plan  of  this  book  to  study  all 
branches  of  chemical  theory.  Only  such  will  be  taken 
up  as  bear  directly  upon  the  question  of  the  constitution 
of  matter.  It  will  be  found,  however,  that  this  includes 
most  of  the  important  theories. 

Where  a  great  science  is  founded  upon  a  theory,  in  so 
far  as  the  explanation  of  its  facts  are  concerned,  it  is 
fitting  for  those  who  love  that  science,  and  higher  still, 
love  truth,  to  examine  well  its  foundation,  to  trace  it 
back  to  its  far-off  inception  and  to  test  it  by  all  wise  and 
skilful  methods  for  finding  out  the  truth,  feeling  assured 
that  only  good  can  come  from  such  examination.  It  will 
strengthen  them  to  know  how  sure  is  their  foundation,  or 
if  it  be  found  unstable,  it  will  be  wise  to  discard  it  before 
more  harm  is  done.  For  the  false  can  not  lead  up  to  truth 
and  it  is  toward  truth  and  truth  alone  that  the  labors  of 
all  students  of  Nature  should  tend* 


CHAPTER  I. 


Ancient  Views  as  to   the   Nature  of 
Matter. 


CHAPTER  I. 
ANCIENT  VIEWS  AS  TO  THE  NATURE  OF  MATTER. 

„,.      „  A   theory   as  to   the  constitution  of 

The  Constitu-  .  f        _ 

tion  of  Matter.  matter  is  an  effort  at  making  clear  the 
conditions  upon  which  rest  the  filling 
of  space  with  distinguishable  entities  and  the  change  of 
these.  The  history  of  such  theories  is  closely  connected 
with  the  development  of  all  science,  for  its  object  em- 
braces the  entire  content  of  all  experience.1  From  the 
very  earliest  time  man  has  taken  a  strong  theoretical  in- 
terest in  the  material  world  and  developed  from  a  meta- 
physical standpoint  his  views  as  to  the  world  formation.* 
That  this  theoretical  interest  preceded,  however,  that 
practical  interest  in  nature  which  led  to  its  subjugation 
so  as  to  supply  immediate  necessities  can  scarcely  be 
maintained .  The  technical  man  preceded  the  philosopher, 
the  needs  of  daily  life  were  first  supplied. 

~.  The  theories  of  chemistry   take  their 

Theory  and  .  : 

Empiricism.  nse  ln  ^e  cosmogonies  of  the  philoso- 
phers or  their  efforts  at  accounting  for 
the  origin  and  building  of  the  universe.  Experimental 
chemistry,  on  the  other  hand,  was  at  first  eminently 
practical  and  was  derived  from  the  empirical  knowledge 
of  the  metal  worker,  the  potter,  the  cook  and  the 
dyer.  It  is  to  be  expected  then  that  the  technical  man 
should  altogether  disregard  the  theories  of  the  meta- 
physician as  to  the  world  around  him,  or  that  he  should 
use  only  so  much  of  these  theories  as  come  well  within 
liis  experience.  Thus  the  corpuscular  theory  of  the 

1  Lasswitz  :  "Geschichte  der  Atomlehre,"  i,  x. 

2  I^asswitz,  i,  6. 


6  A  STUDY  OF  THE  ATOMS. 

mechanician,  Hero  of  Alexandria,  and  of  the  physician, 
Asclepiades  of  Bithynia,  stood  in  close  relation  to  the 
atomic  theory  of  Democritus,  and  yet  was  not  that  theory. 

It  is  the  purpose  of  this  book  to  trace  the 

rise   and   development    of    one  of  these 
the  Book. 

theories   which   sprung    from    the   early 

cosmogonies — its  inception  by  the  meta-physician  and  its 
tardy  acceptation  by  the  man  of  experiments  after  he  had 
satisfied  himself  that  it  agreed  with  his  experience  and 
afforded  the  most  satisfactory  explanation  of  that  experi- 
ence. This  theory  has  been  known  for  some  twenty- 
four  hundred  years  as  the  atomic  theory.  The  field  in- 
cludes, then,  man's  study  of  the  atoms,  speculative  and 
experimental,  from  the  dawn  of  science  to  the  present  day. 

Without  going  into  a  detailed  development 
The  Two  of  them  at  presenti  it  may  be  stated  that 

there  are  two  possible  theories  as  to  matter. 
One  is  that  matter  is  infinitely  divisible  and  that  no  limit 
can  be  placed  to  the  possibility  of  its  subdivision.  The 
other  is  that  matter  is  not  infinitely  divisible,  but  that 
eventually  particles  will  be  reached  which  are  no  further 
divisible  by  any  known  means.  Such  particles  were 
called  atoms  by  the  Greek  philosophers  and  this  theory 
became  known  as  the  atomic  theory. 

Probably  the  most  ancient  document  ex- 
tant  containm£  reference  to  this  idea  is  the 
Shoo  King,1  which  is  one  of  the  oldest  and 
most  esteemed  of  Chinese  classics,  and  here  the  idea 
is  rather  that  of  elementary  particles  than  of  atoms.  This 
treatise  is  an  historical  work  and  comprises  a  document  of 

i  Cited  by  Gladstone  in  Address  to  Chemical  Section,  British  Association, 
1883.     Chem.  News,  48,  151  (1883). 


ANCIENT  VIEWS.  7 

still  greater  antiquity  called  "The  Great  Plan  with  Its 
Nine  Divisions."  This  purports  to  have  been  "given  by 
Heaven  to  the  Great  Yu  to  teach  him  his  royal  duty  and 
the  proper  virtues  of  the  various  relations."  Of  course 
there  is  no  perfect  agreement  as  to  the  date  of  this  docu- 
ment and  the  opinions  vary  widely  concerning  it,  but  it 
seems  fairly  safe  to  assign  it  to  a  time  more  ancient  than 
the  writings  of  Solomon.  The  first  division  of  the  Great 
Plan  relates  to  the  five  elements.  The  first  element  is 
named  water;  the  second  fire;  the  third  wood ;  the  fourth 
metal ;  the  fifth  earth,. 

Without  attempting  to  settle  the  question 

*n  °.°  of  priority  in  this  conception  of  primal  ele- 

i  neories.  .  . 

ments  it  is  sufficient  to  state  that  a  similar 

idea  is  found  in  the  early  literature  of  several  nations, 
notably  among  the  Indian  races,  though  the  number  and 
names  of  the  elements  may  differ. 

In  the  Institutes  of  Menu  the  subtle  ether  is  spoken  of 
as  being  the  first  created.  From  this,  by  transmutation, 
came  air  and  this  through  some  change  became  light  or 
fire,  and  by  a  further  change  in  this  came  water  from 
which  lastly  earth  is  deposited.  This  was  the  accepted 
philosophy  of  the  Hindoos  and  Buddhists.  It  extended 
over  Asia  and  found  its  way  into  Kurope.  It  has  been 
claimed  that  it  was  elaborated  in  the  philosophy  of  the 
Greeks.1 

These  early  ideas  as  to  primal  elements  would  seem  to 
have  little  or  no  bearing  upon  the  theory  of  atoms.  In 
thinking  of  the  genesis  of  matter,  however,  the  first 
thought  was  as  to  the  primal  element  or  elements  and  the 
conception  of  the  atom  was  most  probably  evolved  from 
this  idea. 

1  Gladstone  :  Loc.  cit. 


8  A  STUDY   OF  THE   ATOMS. 

It  is  exceedingly  difficult  to  interpret 
Atomic  Theory  aright  many  of  the  obscure>  ancient 
ol  ivana.ua.  . 

writings  of  China  and  India.     Yet  in 

these  literatures  definite  traces  of  the  theory  of  atoms  can 
be  distinguished.  Thus  an  atomic  theory  has  been  pro- 
posed by  Kanada,  the  founder  of  the  Nyaya  system  of 
philosophy,  of  which  this  theory  forms  a  distinguishing 
feature.1  First  as  to  the  elements  it  is  stated  by  Kapila, 
founder  of  the  Samkhya  philosophy,  that  there  are  five 
subtle  particles,  rudiments  or  atoms,  perceptible  to  beings 
of  a  superior  order  but  unapprehended  by  the  grosser 
senses  of  mankind,  derived  from  the  conscious  principle 
and  themselves  productive  of  the  five  grosser  elements, 
earth,  water,  fire,  air  and  space. 

Kanada  considered  material  substances  to  be  primarily 
atoms,  secondarily  aggregate.  He  maintains  the  eternity 
of  atoms. 

' '  The  mote  which  is  seen  in  a  sunbeam  is  the  smallest 
perceptible  quantity  ;  being  a  substance  and  an  effect,  it 
must  be  composed  of  what  is  less  than  itself.  This  again 
must  be  composed  of  what  is  smaller,  and  that  smaller 
thing  is  an  atom.  It  is  simple  and  uncomposed  else  the 
series  would  be  endless  ;  and  were  it  pursued  indefinitely 
there  would  be  no  difference  of  magnitude  between  a 
mustard  seed  and  a  mountain,  each  alike  containing  an 
infinity  of  particles.  The  ultimate  atom  then  is  simple. 

' '  The  first  compound  then  consists  of  two  atoms,  the 
next  consists  of  three  double  atoms.  Two  earthly  atoms 
concurring  by  an  unseen  virtue,  creative  will  of  God,  or 
other  competent  cause,  constitute  a  double  atom  of  earth 
and  by  concourse  of  three  binary  atoms  a  tertiary  atom  is 
produced.  The  atom  is  reckoned  to  be  the  sixth  part  of  a 

i  "  History  of  Hindu  Chemistry,"  Ray,  5,  6. 


ANCIENT  VIEWS.  9 

mote  visible  in  a  sunbeam.  The  atoms  are  eternal,  the 
aggregates  are  not.  The  aggregates  may  be  organized 
organs  and  inorganic."  No  definite  date  can  be  assigned 
to  Kanada  but  he  seems  to  have  lived  before  the  time  of 
Democritus. 

According  to  another  view  the 

<-  »«*•  - » ** «—- 

tution  of  matter  have  been 
derived  from  mathematical  considerations  as  to  number, 
time  and  space,  and  not  deduced  from  theories  as  to  the 
genesis  of  matter.  This  is  ingeniously  worked  out  as 
follows  :l 

It  is  probable  that  the  first  exact  notions  of  quantity 
were  founded  on  the  consideration  of  number.  It  is  by 
the  help  of  numbers  that  concrete  quantities  are  prac- 
tically measured  and  calculated.  Now  number  is  dis- 
continuous. We  pass  from  one  number  to  the  next  per 
saltum.  The  magnitudes,  on  the  other  hand,  which  we 
meet  with  in  geometry  are  essentially  continuous.  The 
attempt  to  apply  numerical  methods  to  geometrical 
quantities  led  to  the  doctrine  of  incommensurables  and  to 
that  of  the  infinite  divisibility  of  space.  Meanwhile  the 
same  considerations  had  been  applied  to  time  so  that  in 
the  days  of  Zeno  of  Elea  time  was  still  regarded  as  made 
up  of  a  finite  number  of  *  *  moments, ' '  while  space  was 
confessed  to  be  divisible  without  limit. 

Aristotle  pointed  out  that  time  is  divisible  without  limit 
in  precisely  the  same  sense  that  space  is.  It  was  easy  to 
attempt  to  apply  similar  arguments  to  matter.  If  matter 
is  extended  and  fills  space  the  same  mental  operation  by 
which  we  recognize  the  divisibility  of  space  may  be 
applied,  in  imagination  at  least,  to  the  matter  which 

1  Clerk-Maxwell,  article  on  "  Atoms"  in  Encyc.  Brit. 


10  A  STUDY   OF  THE   ATOMS. 

occupies  space.  From  this  point  of  view  the  atomic  doc- 
trine might  be  regarded  as  a  relic  of  the  old  numerical 
way  of  conceiving  magnitude  and  the  opposite  doctrine 
of  the  infinite  divisibility  of  matter  might  appear  for  the 
time  the  most  scientific.  The  atomists,  on  the  other 
hand,  asserted  very  strongly  the  distinction  between 
matter  and  space.  The  atoms,  they  said,  do  not  fill  up 
the  universe.  There  are  void  spaces  between  them.  If 
it  were  not  so,  Lucretius  tells  us,  there  could  be  no 
motion,  for  the  atom  which  gave  way  must  have  some 
empty  space  to  move  into.  It  would  be  better,  however, 
to  postpone  an  account  of  the  arguments  along  this  line 
until  the  views  of  the  early  Greek  philosophers  have  been 
studied. 

By  far  the  fullest  and  clearest  theories  have 
Greek 
T«        .  come  to  us  from  the  Greeks.      How  far 

these  originated  with  them  it  is  difficult  to 
say  and  the  point  has  been  vigorously  discussed  without 
reaching  any  satisfactory  conclusion.  Gladisch1  sees  in 
the  Pythagorean  theories  the  philosophy  of  the  Chinese  ; 
that  of  the  Hindoos  in  the  Eleatics  ;  that  of  the  Persians 
in  Heraclitus  (Lasalle  maintains  that  Heraclitus  de- 
rived his  philosophy  from  India)  ;  that  of  the  Egyptians 
in  Empedocles  and  that  of  the  Jews  in  Anaxagoras.  The 
truth  would  rather  seem  to  be  that  there  were  racial 
ideas  held  in  common.  The  Greeks,  coming  from  Asia 
as  did  the  other  Indo- Germanic  races,  brought  these 
theories  with  them,  modified  them  according  to  their  own 
peculiar  conditions  and  environment  and  developed  them 
by  their  own  powers.  Burnet  regards  the  idea  of  the 
introduction  of  Eastern  philosophy  into  Greece  as  fanci- 
ful and  probably  a  suggestion  of  Egyptian  priests  and 

i  Zeller  :  "Pre-Socratic  Phil.,"  i,  35. 


ANCIENT  VIEWS.  II 

Alexandrian  Jews.1    Indian  Science,  it  has  been  claimed, 
came  from  Greece  in  the  train  of  Alexander's  army. 

It  is  not  altogether  easy  to  form  a  correct  idea  as  to 
the  theories  of  the  earliest  of  the  Greek  philosophers  since 
we  are  largely  dependent  upon  the  record  and  interpre- 
tation of  them  given  in  the  writings  of  later  followers, 
antagonists,  or  lexicographers.  In  the  case  of  many  of 
them  only  scattered  fragments  of  their  writings  have 
been  preserved.  We  will  consider  the  views  of  these 
philosophers  in  detail. 

The  earliest  Greek  cosmogonists  were  those 
of  the  Ionic  school.  These  men  were  char- 
acterized by  their  love  of  knowledge  and  their 
diligent  search  for  it  everywhere.  They  were  not  satis- 
fied with  the  mere  observation  of  phenomena  but  sought 
for  law  in  everything  and  strove  to  construct  systems  of 
the  universe.  To  find  out  the  genesis  of  all  things  was  a 
fascinating  thought  to  them.  In  the  midst  of  the  changes 
surrounding  them,  especially  those  of  generation  and 
decay,  they  sought  for  something  primeval  and  un- 
changing. Burnet  has  pointed  out2  that  their  word  (pv6i$, 
from  which  our  word  physics  comes,  was  used  by  the 
early  cosmogonists  to  express  the  idea  of  a  permanent 
and  primary  substance  so  that  nepi  (pvffecos  does  not 
mean,  as  ordinarily  translated,  "On  the  Nature  of 
Things"  but  "Concerning  the  Primary  Substance." 
There  are  traces  of  a  careful  and  minute  investigation  of 
nature  by  these  philosophers  in  search  of  evidence  to  sup- 
port their  theories.  Thus  Xenophanes,  to  substantiate 
certain  of  his  views,  made  a  careful  investigation  of  the 
fossils  and  petrifactions  in  such  widely  separated  localities 
as  Paros,  Malta  and  Syracuse. 

1  Burnet :  "Early  Greek  Phil.,"  15. 

2  Burnet :  "Early  Greek  Phil.,"  10. 


12  A  STUDY  OF  THE   ATOMS. 

The  founder  of  the  Ionic  school  was  Thales 

Miletus  °f  Miletus  who  lived  about  6o°  B-  c-  He 

600  B.  C.  *e^  no  writing8  and  we  are  dependent  upon 
Aristotle  for  our  knowledge  of  his  views. 
These  he  gives  in  three  statements :  (i)  The  earth  floats 
on  water;  (2)  Water  is  the  material  cause  of  all  things  ; 
(3)  All  things  are  full  of  gods,  and  the  magnet  is  alive,  for 
it  has  the  power  of  moving  iron.  His  view  of  the  universe 
would  seem  to  be  a  space  filled  with  a  fluid,  water,  and 
out  of  this  principle  the  solid  earth  and  all  things  upon  it 
were  formed.  These  he  endowed  with  life,  and  following 
him  all  the  Ionic  philosophers  were  hylozoists.  He  was 
ignorant  of  the  atmosphere  or  air,  the  arjp  of  Homer 
meaning  first  the  mist  or  vapor  such  as  that  rising  from 
the  ocean. 

Nearly  all  that  is  known  of  the  next 
Aiuiximander,  of  these  philosophers,  Anaximander 

(also  of  Miletus,  546  B.  C.),  is  from 
the  account  given  by  Theophrastus.  According  to  him, 
Anaximander  maintained  that  neither  water  nor  any  simi- 
lar substance  was  the  primal  element  but  a  substance  dif- 
fering from  any  of  them  and  merely  described  as  infinite. 
Into  that  from  which  things  took  their  rise  they  passed 
away  once  more.  The  origin  of  things  was  not  due  to 
any  alteration  in  matter  but  to  the  separation  of  ' '  oppo- 
sites  from  the  boundless  substratum."  There  was  an 
eternal  motion  in  the  course  of  which  was  brought  about 
the  origin  of  worlds.  His  theory  then  was  one  of  the 
Boundless  or  Infinite,  one  eternal,  indestructible  sub- 
stance. He  saw  no  up  nor  down  in  nature. 

.        .  Anaximenes,   who  is  spoken  of  as  an 

associate   of   Anaximander,    said   also 

that  the  substratum  was  one  and  infinite.     He  did  not, 


ANCIENT  VIEWS.  13 

however,  consider  it  indeterminate  in  character  but  called 
it  air,  meaning  by  this,  probably,  vapor  or  mist.  It  is 
always  in  motion  and  differs  in  different  substances  in  vir- 
tue of  rarefaction  and  condensation.  The  introduction 
of  the  idea  of  rarefaction  and  condensation  constitutes  a 
distinct  advance  as,  where  everything  is  formed  by  the 
transformation  of  one  substance,  all  differences  must  be 
purely  quantitative.  The  previous  theories  are  incom- 
plete and  impossible  unless  diversities  are  considered  as 
due  to  the  presence  of  more  or  less  of  the  materia  prima 
in  a  given  space. 

The  philosophy  of  Pythagoras  and  the 
Pythagorean  Pythagorean  school  was  of  a  religious 

and  political  character  rather  than  phys- 
ical. It  is  also  largely  mathematical  and  distinct  from 
the  theories  of  the  early  cosmogonists.  L,ittle  is  known 
with  certainty  as  to  Pythagoras  himself  and  his  beliefs, 
but  the  views  of  the  school  founded  by  him  have  in  them 
the  germ  of  some  later  theories  of  science.  The  distinc- 
tive feature  of  the  Pythagorean  school  was,  that  number 
is  the  essence  of  all  things  and  everything  in  its  essence 
is  number.1  Numbers  are  not  merely  qualities  of  things 
but  the  substance  of  things.  While  numbers  are  regarded 
by  us  only  as  the  expression  of  the  relation  of  substance 
they  thought  that  they  found  in  them  the  substance  or  the 
real.  All  numbers  are  divided  into  the  odd  and  even  and 
to  these  the  third  class,  the  even-odd  (dpriOTtepiaffov} 
was  added.  Everything  united  in  itself  opposite  character- 
istics, the  odd  and  even,  the  limited  and  unlimited.  The 
primary  constituents  of  a  thing,  therefore,  are  dissimilar 
and  opposite  in  character.  The  uniting  bond  is  harmony.* 

i  Aristotle's,  Metaphys.  I,  5. 
«  Philolaus  Ap.  Stob.,  I,  460. 


14  A   STUDY  OF  THE   ATOMS. 

According  to  Eudorus1  the  Pythagoreans  reduced  all 
things  ultimately  to  the  one,  or  unity.  This  is  the  first 
principle,  the  efficient  cause  of  all  things.  Duality  is 
passive  matter.  Unity,  or  perfection,  as  opposed  to 
duality,  or  imperfection  was  called  the  monad.  It  is  quite 
difficult  to  decide  whether  the  Pythagoreans  regarded 
their  numbers  as  something  corporeal,  or  bodies  as  some- 
thing immaterial.  Still,  whether  from  a  false  interpreta- 
tion or  not,  their  theory  has  come  to  be  regarded  as  the 
theory  of  the  harmony  of  nature,  its  essential  oneness 
and  the  derivation  of  all  things  from  one,  a  sublime 
thought  in  itself  and  one  which  has  played  a  large  part 
in  the  philosophy  of  the  past  centuries  and  again  comes 
into  prominence  in  the  speculations  of  the  present  day. 

Anaxagoras,  of  Klazomene,  is  one  of  the 
first  of  the  philosophers  whose  views 
approximated  to  an  atomic  theory. 
Others  who  had  preceded  him  had  advanced  theories  as  to 
the  primal  elements  but  not  as  to  the  internal  constitution 
of  matter.  According  to  the  theory  of  Anaxagoras  there 
was  first  a  chaos.  All  matter  was  in  the  form  of  mingled 
particles  in  infinite  disorder.  These  particles  were  called 
by  Aristotle  in  later  times  homoeomerous  (ojioiojtspeiai) 
which  means  ' '  like  parts. ' '  That  is,  these  very  small 
particles  were  similar  to  the  masses  of  matter  afterwards 
formed  by  their  aggregation.  They  were  rather  mole- 
cules than  atoms.  The  vovs,  or  designing  intelligence, 
brought  these  out  of  chaos  and  formed  of  them  matter  as 
known  to  us.  Anaxagoras'  statement  that  there  is  a  por- 
tion of  everything  in  each  particle  is  best  explained  as 
meaning  that  in  each  was  to  be  found  a  portion  of  the 
qualities  moist  and  dry,  hot  and  cold,  light  and  dark,  but 

i  Simpl.  Phys.,  39.  A. 


ANCIENT  VIEWS.  15 

the  predominating  portion  determined  the  character  of 
the  particles.  Among  these  particles  the  rov5  began  a 
rotary  motion  through  which  the  like  particles  were 
gradually  brought  together  and  separated  out  in  the  form 
of  the  various  known  substances.  As  Rodwell  says,  this 
vertical  motion  was  supposed  to  have  drawn  together  the 
similar  homoeomerous  by  a  process  something  like  that  of 
gathering  the  gold  grains  in  a  pan  during  the  process  of 
washing.1  In  other  words,  this  acted  very  much  as  water 
does  in  sorting  out  substances  of  different  specific  gravi- 
ties. According  to  Anaxagoras  then,  on  dividing  a 
body,  as  a  grain  of  earth,  particles  of  earth  are  obtained. 
This  can  be  continued  indefinitely  and  still  the  grains  re- 
semble the  original  grain  of  earth.  There  is,  therefore, 
no  limit  to  such  subdivision.  This  became  later  the 
philosophy  of  the  Peripatetics  or  Pythagoreans,  Such 
particles  are  manifestly  not  atoms  which  were  not  neces- 
sarily like  the  mass  and  were  indivisible.  To  Anax- 
agoras bone  was  made  up  of  minute  particles  of  bone, 
blood  of  minute  drops  of  blood,  water  of  minute  drops  of 
water.  This  would  necessitate  the  original  existence  of 
a  distinct  particle  for  every  distinct  kind  of  matter. 

His  introduction  of  an  external  cause  to  produce  the 
motion  is  noteworthy,  but  too  much  stress  must  not  be 
placed  upon  this  mind  or  designing  intelligence  or  disap- 
pointment such  as  that  felt  by  Socrates  or  Aristotle  may 
follow."  Aristotle,  says  Anaxagoras,  used  mind  as  a  deus 
•ex  machina  to  account  for  the  formation  of  the  world  and 
whenever  he  is  at  a  loss  to  explain  anything,  he  drags  it 
in.  But  in  other  cases  he  makes  anything  rather  than 
mind  the  cause.1 

i  Rodwell :  "  Birth  of  Chemistry,"  18. 

8  Plato :  Phaid,  97  B. 

*  Aristotle's  Metaphys.  A.  4  985*. 


1 6  A  STUDY  OF  THE  ATOMS. 

According  to  Heraclitus,  the  next  one 

sooB ;  c"8'       of  these Philos°Phers>  a11  thinss are one- 

"It  is  wise  for  those  who  hear,  not  me 
but  the  universal  reason,  to  confess  that  all  things  are 
one."1  He  assumed  as  the  primal  element,  fire.  In  his 
opinion  there  was  nothing  fixed  and  permanent  in  the 
world  but  all  was  involved  in  constant  change  as  the 
waves  of  a  river  are  constantly  replaced  by  those  follow- 
ing.2 The  restless  alteration  of  phenomena  became  com- 
prehensible to  him  by  considering  the  world  a  fire.  Fire 
was  the  life  of  nature.  Everything  was  created  by  fire 
and  was  dissolved  into  fire.  Fire  was  not  an  unvarying 
substance  out  of  which  all  things  were  formed  while  it 
itself  remained  unchanged  qualitatively  like  the  elements 
of  Kmpedocles.  On  the  contrary  it  was  the  essence 
which  passes  ceaselessly  into  all  elements,  the  universal 
nourishing  matter  which,  in  its  eternal  circulation,  per- 
meates all  parts  of  the  cosmos,  assumes  in  each  a  differ- 
ent constitution,  produces  individual  existence  and  again 
resolves  itself  and  by  its  absolute  motion  causes  the  rest- 
less beating  of  the  pulse  of  nature.3  The  elements  of  the 
physicist  were  those  which  amid  the  change  of  particular 
things  remained  unchangeable.  To  Heraclitus,  fire  was 
that  which  by  constant  transmutation  caused  the  change. 
The  harmony  of  the  world  was  due  to  the  strife  of  oppo- 
sites. 

Empedocles  sought  a  middle  course  be- 

oro1IB  c  CS>       tween  Heraclitus  who  said  that  ma*ter 

was   always  changing  and   Parmenides 

who   denied    change,    motion,    generation,   and    decay. 

Qualitative  change  in  the  original  substance  was  to  him. 

1  Patrick :  "Fragments  of  Heraclitus,"  I. 
8  Patrick:  "Fragment*  of  Heraclitus, 
'  Zeller:  "Pre-Socratic  Phil.,"  II,  23. 


ANCIENT  VIEWS.  17 

unthinkable  but  there  is  change  for  particular  things,  and 
the  conditions  of  the  world  are  subject  to  perpetual 
change.  These  phenomena  of  change  he  reduced  to  a 
movement  in  space,  to  the  combination  and  separation  of 
the  underived,  imperishable  and  quantitatively  unchange- 
able substances.  In  this  he  is  the  first  to  clearly  define 
the  elemental  constituents  or  elementary  bodies.  He  had 
to  assume  several  of  these  in  order  to  explain  the  multi- 
plicity of  things.  Aristotle  states1  that  Empedocles  was 
the  first  to  admit  the  four  elements — earth,  air,  fire,  and 
water,  a  theory  which  he  himself  adopted  and  which  was 
generally  accepted  for  centuries.  This  theory,  however, 
in  a  very  similar  form  was  common  to  several  races  long 
before  the  time  of  Empedocles.  Further,  in  his  system, 
the  constancy  of  matter  was  maintained  and  all  vacuum 
denied.  He  did  not  speak  of  a  single  substratum  of  all 
the  elements.  They  were  distinctly  underived.  Nor  did 
he  speak  of  ultimate  atoms.  The  fact  that  his  elements 
were  unchanging  may  be  regarded  as  the  most  important 
advance  in  thought  leading  up  to  the  unchanging  atom. 
The  dualistic  idea  as  to  force  or  controlling  and  directing 
influences  is  to  be  seen  in  the  ' '  Opposites' '  of  Anaxi- 
mander,  the  strife  of  the  opposites  bringing  about  har- 
mony according  to  the  system  of  Heraclitus  (who  gives 
as  instances  the  high  and  low  notes  of  music  and  the  con- 
trasted colors  of  the  painter)  and  in  the  lyight  and  Dark- 
ness of  Parmenides.  It  is  still  more  clearly  brought  out 
in  the  love  and  strife  of  Empedocles.  In  his  system  the 
contending  forces  cause  the  combination  and  separation 
of  the  elements.  There  were  four  cycles  or  spheres.  In 
the  first,  all  the  elements  are  mixed  by  love  ;  in  the  sec- 
ond, love  is  passing  out  and  strife  coming  in  (partial 

i  Aristotle's  Metaphys.  I,  4.    985  a  31. 


1 8  A  STUDY  OF  THE  ATOMS. 

separation  and  partial  combination);  in  the  third,  love 
is  banished  and  there  is  complete  separation ;  in  the 
fourth,  love  is  gradually  bringing  the  elements  together 
again  and  strife  is  passing  away.  Such  a  world  as  ours 
can  exist  in  only  the  second  or  fourth  cycles. 

Leucippus  is  regarded  as  the  founder  of 

the  Atomistic  scn°o1-    while  the  date  and 
place  of  his  birth  are  not  recorded  it  is 

known  that  he  lived  in  the  middle  or  latter  part  of  the 
fifth  century  B.  C.  and  that  he  was  the  contemporary  of 
Anaxagoras,  of  Klazomene,  and  Kmpedocles,  of  Agri- 
gentum.  His  most  famous  pupil  was  Democritus  who 
later  became  his  associate  and  developed  his  philosophy. 
No  writings  of  his  are  known  and  in  the  light  of  the 
greater  fame  of  his  pupil,  Democritus,  he  was  ignored  by 
both  Epicurus  and  Lucretius.  He  is  referred  to  by 
Aristotle. 

According  to  L,eucippus  all  things  consisted  of  empty 
spaces  and  atoms  (aro}io$,  from  a  and  TSJAVSIV,  to  cut), 
space  being  infinite  in  magnitude  and  atoms  infinite  in 
number.  These  atoms  were  further  indivisible,  having 
only  quantitative  differences  between  one  another  and  be- 
ing always  in  motion.  Instead  of  the  vov$  or  designing 
intelligence  of  Anaxagoras,  avdyKrj,  or  necessity,  was 
the  promoting  cause  of  all  things.  Worlds  are  formed 
by  the  falling  together  of  atoms,  varying  in  shape  and 
weight,  in  empty  space,  their  impact  giving  rise  to  a  new 
eddying  motion  and  the  motion  causing  the  formation  of 
all  substances.  These  atoms  were  quite  distinct  from  the 
homoeomerous  of  Anaxagoras,  as  they  were  not  neces- 
sarily similar  to  the  substances  formed  from  them  but  were 
the  "seeds  of  things". 


ANCIENT  VIEWS.  19 

It  is  mainly  through  fragmentary  quotations 

S 'h^T  and  the  statements  of  Aristotle  that  the  works 
of  the  Eleatic  philosophers  are  known  to  us. 
The  most  prominent  of  these  philosophers  are  Xeno- 
phanes,  Zeno  and  Parmenides.  This  philosophy  was 
monotheistic,  believing  in  one  underived  all-embracing 
being.  Change  was  regarded  as  impossible  in  a  univer- 
sal sense  and  they  were  opposed  to  the  idea  of  multi- 
plicity and  plurality.  The  atomists  were  classed  with  or 
included  in  the  Eleatic  school  and  Aristotle  remarks  upon 
the  affinity  existing  between  the  two.1  But  they  differed 
greatly  as  to  motion  and  change,  the  possibility  of  either 
being  denied  by  the  Eleatics.  Zeno's  arguments  against 
the  possibility  of  motion  will  be  introduced  later  as  a 
specimen  of  their  dialectics.  While  the  eternal  oneness 
of  nature  was  maintained,  the  Eleatics  proper  did  nothing 
to  advance  the  doctrine  of  atoms. 

The  theory  of  Leucippus  was  taken  up 
by  Democritus'  of  Abdera,  who  had  been 
his  pupil  and  then  his  associate  but  who 
excelled  his  master  as  a  deep  and  orderly  thinker.  He 
defended  and  developed  the  theory  of  atoms  to  such  an 
extent  that  to  him  is  usually  accredited  the  title  of 
founder  of  the  Atomistic  school.  '  'The  existence  of  atoms 
must  be  admitted,"  he  said,  "because  of  the  principle 
that  nothing  is  made  of  nothing."  "If  every  substance 
is  divisible  to  infinity  and  the  division  is  never  arrested, 
we  come  to  one  of  two  things:  either  nothing  remains  or 
something  is  always  left.  In  the  first  case  the  body  was 
made  up  of  nothing  or  it  was  composed  of  an  apparent 
reality.  In  the  second  case,  one  might  ask,  what  is  it 
that  remains,  an  entity  or  a  space?  But  then  the 

*  Aristotle  :  "Gen.  et  Coir.,"  i,  8. 


20  A   STUDY   OF  THE   ATOMS. 

division  could  not  have  been  exhausted.  Does  a  point 
remain  ?  But  whatever  may  be  the  number  of  points 
which  are  suggested,  it  will  never  fill  a  space.  There- 
fore it  is  necessary  to  admit  the  existence  of  real  in- 
divisible elements."1  This  line  of  reasoning  was  bor- 
rowed from  or  very  similar  to  that  of  Zeno.2  Further, 
Democritus  reasoned  that  the  atoms  varied  not  only  in 
size  and  in  weight  but  the  main  distinction  between  them 
was  in  shape.8  The  smallest  atoms  are  at  the  same  time 
the  lightest.  The  atoms  are  absolutely  simple  and  homo- 
geneous, differing  in  this  from  those  of  Anaxagoras,  and 
are  impenetrable  :  two  atoms  cannot  occupy  the  same 
space  at  the  same  time.  Each  atom  resists  the  atom 
which  tends  to  displace  it.  This  resistance  gives  an 
oscillatory  motion  which  is  communicated  to  neighboring 
atoms  which  transmit  it  to  the  more  distant  atoms.  From 
this  springs  a  gyratory  motion,  a  rotation  which  is  a  type 
of  all  the  motions  in  the  world.4  The  Eleatics  had  formed 
the  concept  that  being  can  only  be  denned  as  indivisible 
unity.  Leucippus  and  Democritus  supposed  the  cor- 
poreal to  be  composed  of  parts  incapable  of  further 
division  ;  all  consists  of  atoms  and  the  void.  All  the 
properties  ascribed  by  the  Eleatics  to  being  are  transferred 
to  the  atoms.5  These  atoms  are  too  small  to  be  perceived 
by  the  senses  since  every  substance  perceptible  to  sense 
is  changeable  and  divisible. 

Little  is  known  as  to  Democritus'  opinion  about  the 
four  elements  of  Empedocles.  Fire  alone  seems  to  have 
had  for  him  any  very  great  importance  among  the  theories 
of  primal  elements.  He  considered  fire  the  moving, 

»  Aristotle:  "Gen.  ct.  Cor.,"  I,  e,  2,  8. 

*  Simplicius  Phys.  3o,a. 
»  Aristotle,  Phys.  I.,  a. 

«  Plutarch:  "de  Placit  philos.,"  I.,  26  ;  Stobae:  "Bclog.  phys.,"  I.,  394. 

•  Zeller:  "Pre-Socratic  Phil.,"  II. ,  219. 


ANCIENT   VIEWS.  21 

living  principle  throughout  nature.  On  account  of  its 
mobility  he  supposed  it  to  consist  of  round  and  small 
atoms.  In  the  other  elements  there  is  a  mixture  of  hetero- 
geneous atoms  and  they  are  distinguished  from  one 
another  only  by  the  magnitude  of  their  parts.1  The 
atoms  are  in  ceaseless  movement,2  which  was  so  necessi- 
tated by  the  nature  of  things  that  he  considered  it  to  be 
without  beginning.3  This  movement  was  a  result  of  their 
weight.  The  movement  of  all  atoms  would  be  in  the 
same  direction.  The  inequalities  in  size  and  weight 
bring  about  unequal  velocities.  They  impinge  upon  one 
another  and  the  lighter  are  forced  upward  by  the  heavier. 
From  the  resultant  of  these  two  motions,  the  concussion 
and  recoil  of  the  atoms,  there  arises  a  circular  or  whirling 
movement.  From  this  circular  motion  the  universe  was 
derived.  Through  this  movement  of  the  atoms,  homo- 
geneous particles  are  brought  together,  being  alike  in 
weight  and  form,  and  so  sink  into  the  same  place.4  From 
the  combination  of  atoms,  compound  bodies  are  formed. 
The  atoms  he  thought  to  be  infinite  in  number  and  in- 
finitely various  in  form  and  size.  Democritus  and  the 
atomists  endeavored  to  give  a  strictly  physical  and  mate- 
rial explanation  of  nature.  Nothing  happened  by  chance  ; 
all  could  be  referred  to  natural  causes.  Democritus  has 
been  spoken  of  as  an  empiricist  rather  than  a  philosopher. 
Certainly  he  devoted  more  attention  to  the  explanation  of 
natural  phenomena  than  any  of  his  predecessors  and  quite 
possibly  he  accumulated  more  empirical  material  than  he 
was  able  to  master  with  his  scientific  theory.  He  did 
not  neglect  experimental  science  and  sought  in  actual 
knowledge  of  things  a  basis  for  his  theories.  His  system 

1  Zeller:  "Pre-Socratic  PhiL"  IL,  234. 
»  Aristotle's  "Metaphys.,"  XII.,  1070. 
»  Cicero,  Pi,  I.,  6,  7. 
«  Sextus:  Math.  VII.,  116. 


22  A  STUDY   OF  THE   ATOMS. 

is  throughout  materialistic,  dispensing  with  all  save  cor- 
poreal being  and  all  force  save  gravity.  < 

Plato,  in  his  teachings  as  to  world  forma- 
B*  C  t*on'  Deemed  ft  necessary  to  assume  the 
existence  of  the  four  elements  of  Empedo- 
cles.  In  his  physical  derivation  of  these  he  makes  use  of 
the  theory  of  Philolaus,  assigning  geometrical  forms  to 
the  elements  from  considerations,  as  he  says,  of  their 
mobility,  magnitude,  weight,  penetrating  power,  etc. 
The  fundamental  form  assigned  to  fire  is  the  tetrahedron 
(Democritus  considered  the  fire  atoms  spherical  because 
of  their  mobility) ;  of  air,  the  octahedron  ;  of  water,  the 
icosahedron  ;  of  earth,  the  cube.1 

All  superficies,  he  says,2  consist  of  triangles  and  all 
triangles  arise  out  of  the  two  different  right-angled 
triangles,  the  isosceles  and  the  scalene.  Out  of  six  scalene 
triangles  arises  an  equilateral  triangle  and  out  of  four 
isosceles  triangles  arises  the  square  ;  out  of  the  square  is 
formed  the  cube  ;  out  of  equilateral  triangles  the  three 
remaining  bodies.  From  this  it  may  be  seen  that  his 
groundwork  was  space  and  the  atoms,  not  matter  filling 
space  but  certain  parts  of  space  mathematically  limited 
and  comprehended  in  definite  figures.3 

The  properties,  combinations,  decompositions  and  other 
changes  of  these  elements  Plato  discusses  at  length.  His 
theory  is  really  one  of  the  continuity  of  matter  which 
being  space  itself  fills  all  space.  But  he  overlooked  or 
disregarded  certain  difficulties  pointed  out  by  subsequent 
philosophers.  For  instance,4  the  four  elementary  forms 
chosen  by  him  can  never  fill  up  any  space  so  as  to  leave 

i  Plato  55,  D. 

*  Plato  53,  C. 

3  Zeller:  "Plato  and  the  older  Academy,"  374. 

<  Aristotle:  "de  Coelo,"  III.  8. 


ANCIENT  VIEWS.  23 

no  intermediate  space,  nor  can  a  sphere  (the  supposed 
form  of  space)  ever  be  entirely  filled  by  rectilinear  figures, 
and  lastly  the  dissociation  of  an  element  into  the  triangles 
of  which  it  was  composed  must  produce  a  void  as  there 
was  nothing  between  these  triangles. 

The  only  Platonist  whose  views  are  novel 
Heracleides.  enough  to  make  them  suitable  for  cita- 
tion here  is  Heracleides.  While  he  may 
be  considered  a  follower  of  Plato,  he  made  some  note- 
worthy divergences  from  his  doctrine.  He  assumed  as 
the  primary  constituents  of  all  things  minute  bodies, 
themselves  not  compound  nor  made  out  of  anything  else. 
These  atoms  differed  from  those  of  Democritus  in  that 
they  were  supposed  to  be  capable  of  affecting  or  influenc- 
ing one  another.  This  was  not  a  mechanical  influence, 
but  one  of  actual  interdependence. 

The  most  famous  of  Greek  philosophers  and 

tbe  °ne  wll°  exerciseci  tlie  greatest  influence 
upon  subsequent  thought  was  Aristotle. 
In  his  eighteenth  year  he  entered  the  school  of  Plato,  at 
Athens,  and  continued  in  it  until  the  death  of  the  master, 
twenty  years  later.  The  effect  of  this  could  not  fail  to  be 
great  upon  the  philosophic  system  of  Aristotle,  although 
he  saw  the  weak  points  of  his  teacher  and  in  after  years 
criticized  them  unsparingly.  His  own  followers  became 
known  as  the  Peripatetics. 

Aristotle  did  not  believe  in  a  vacuum  or  void.  He  had 
defined  space  as  the  limit  of  the  surrounding  body  in  re- 
spect to  that  which  it  surrounds.1  There  is  then,  no 
space  where  there  is  no  body  as  empty  space  would  be  an 
enclosure  enclosing  nothing.  This  was,  of  course,  directly 
contrary  to  the  teachings  of  the  atomists.  He  further 

1  Aristotle :  "de  Coelo,"  IV.,  3,  310,  6,  7. 


24  A  STUDY  OP  THE   ATOMS. 

differed  from  the  atomists  in  asserting  that  there  was  a 
qualitative  distinction  between  sorts  of  matter,  a  qualita- 
tive alteration  of  material,  and  that  there  might  be  such  a 
combination  of  materials  as  to  cause  the  change  of  their 
qualities. 

Aristotle  opposed  the  idea  of  infinitely  small  bodies. 
He  pointed  out  conclusively  the  fallacies  of  the  Platonic 
system.  How,  for  instance,  can  surfaces  which  have  no 
weight  unite  to  form  bodies  which  have  ?  He  could  not 
regard  it  as  proved  by  Democritus  that  everything  could 
be  deduced  from  a  primal  homogeneous  matter. 

It  is  interesting  to  see  how  Aristotle  derives  his  most 
conclusive  argument  against  the  homogeneity  of  matter 
from  the  phenomenon  of  gravity.  Democritus,  like  Aris- 
totle, was  ignorant  that  all  bodies  mutually  attract  each 
other,  that  within  the  terrestrial  influence  they  all  gravi- 
tate towards  the  center  of  the  earth,  that  the  inequality 
of  the  rate  of  their  descent  is  caused  by  the  resistance  of 
the  air,  and  that  the  pressure  of  the  atmosphere  causes 
the  ascent  of  fire  (heated  gases),  vapors,  etc.  We  have 
become  so  accustomed  to  the  traditional  and  conventional 
views  of  nature  that  it  is  difficult  for  us  to  comprehend 
the  point  of  view  of  these  earlier  philosophers  or  to  see 
the  puzzling  questions  which  surrounded  them.  Democ- 
ritus believed  that  all  the  atoms  fall  downward  in  the 
void,  but  that  the  greater  fall  more  quickly  than  the  less, 
deducing  from  this  hypothesis  the  concussion  of  the 
atoms  and  the  pressure  by  which  the  lesser  are  driven 
upwards.  For  the  same  reason  he  held  that  the  weight 
of  composite  bodies,  supposing  their  circumference  equal, 
corresponds  to  their  magnitude  after  subtraction  of  the 
empty  interstices.  Aristotle  demonstrates  that  this  hy- 
pothesis is  false  :  there  is  no  above  nor  beneath  in  infinite 


ANCIENT  VIEWS.  25 

space,  and  consequently  no  natural  tendency  downwards; 
all  bodies  must  fall  with  equal  rapidity  in  a  void,  nor  can 
the  void  within  bodies  make  them  lighter  than  they 
really  are.  But  Aristotle  goes  farther  and,  ignorant  of  the 
actual  phenomena  which  have  to  be  explained,  rejects 
altogether  Democritus'  theory  of  empty  space,  a  theory 
which  could  not  be  verified  by  the  factors  known  to  an- 
cient science  but  the  foremost  feature  in  the  speculative 
theory  of  Democritus.  He  looked  upon  the  fact  that 
certain  bodies  always  tend  upwards,  rising  more  quickly 
with  increasing  bulk,  as  a  phenomenon  quite  inexplic- 
able on  the  hypothesis  of  absolute  homogeneity  of  mat- 
ter. For,  if  all  bodies  were  composed  of  the  same 
matter,  all  would  be  heavy  and  nothing  light  in  itself. 
Although  it  may  be  that  of  two  bodies  of  equal  size,  the 
denser  might  be  the  heavier,  nevertheless  a  great  mass 
of  air  or  fire  would  necessarily  be  heavier  than  a  small 
quantity  of  earth  or  water,  a  view  which  he  regarded  as 
impossible.  If  gravity  be  determined  by  bulk,  then  a 
great  mass  of  rarer  material  would  be  heavier  than  a 
small  one  of  denser  and  accordingly  would  move  down- 
wards. If,  on  the  contrary,  it  is  said  that  the  more 
vacuum  a  body  contains  the  lighter  it  is,  it  may  be 
answered  that  a  great  mass  of  denser  and  heavier  sub- 
stance includes  more  vacuum  than  a  small  one  of  the 
rarer  sort.  Finally,  if  the  weight  of  every  body  corre- 
sponded to  the  proportion  between  its  bulk  and  the 
empty  interstices,  a  great  lump  of  gold  or  lead  might 
sink  no  faster  than  the  smallest  quantity  of  the  same 
stuff. 

He  concludes  that  we  are  driven  to  assume  the  exist- 
ence of  bodies  heavy  or  light  in  themselves,  which  move 
respectively  toward  the  center  or  circumference  of  the 


26  A   STUDY   OF  THE   ATOMS. 

world  ;  and  this  is  possible  only  when  we  conceive  them 
as  differing  qualitatively  and  not  merely  by  the  figure  or 
magnitude  of  the  elementary  ingredients.1 

Not  only,  in  his  opinion,  did  the  materials  of  the  world 
differ  qualitatively,  but  they  were  subject  to  qualitative 
transformations.  Unless  this  was  admitted,  the  apparent 
transmutation  of  matter  must  be  explained  by  a  simple 
expulsion  of  existing  materials  (Empedocles  and  the 
atomists)  or  by  a  change  in  the  figures  of  the  ele- 
ments (Plato).  The  change  of  water  into  steam  was,  in 
the  theory  of  Aristotle,  a  transmutation  of  the  elements, 
a  qualitative  change  of  material.  Otherwise  he  could 
not  explain  the  great  change  of  bulk  if  the  steam  had 
previously  existed  in  the  water  without  change  or  differ- 
ence. This  formation  of  steam  from  water  was  a  difficult 
problem  to  the  atomists  and  could  not  possibly  be  ex- 
plained by  them  on  the  ground  of  increased  repulsion  of 
the  atoms  or  their  lessened  cohesion  as  in  the  modern 
theory  because  the  atoms  of  Democritus  were  inca- 
pable of  any  internal  change.  Empedocles  and  Anax- 
agoras  explained  steam  as  a  kind  of  air  emanating  from 
water,  and  the  atomists  looked  upon  it  as  a  complex  of 
atoms  escaping  from  water  in  which  they  had  been  pre- 
viously imprisoned.  Of  course,  this  theory  would  leave 
an  untransformed  remnant  which  did  not  accord  with  ex- 
perience. Aristotle  then  rejected  the  existence  of  the 
indivisible  and  of  voids.  He  did  not  regard  a  combination 
(ffvvOsffis)  of  bodies  as  an  absorption  of  one  sort  of 
matter  into  another,  nor  a  merely  mechanical  union  or 
junction  as  the  atomists  did.  When  two  materials  then 
combine,  neither  of  them  remains  the  same  ;  they  are  not 
merely  blended  in  invisible  minute  particles  but  both 
have  passed  wholly  into  a  new  material  wherein  they  re- 

1  Zeller :  "Aristotle  and  the  Earlier  Peripatetics,"  I,  447,  et  seq. 


ANCIENT  VIEWS.  27 

main  potentially  inasmuch  as  they  can  be  again  extracted 
from  it.1 

Epicurus  founded  one  of  the  most  distinc- 

3  2  tive  and  lastinS  of  the  Greek  schools  of 

philosophy.  He  received  instruction  in  the 
system  of  Democritus  and  Plato  and  was  acquainted  with 
the  writings  of  the  chief  philosophers  who  had  preceded 
him.  From  these  he  borrowed  important  parts  of  his 
doctrines,  but  his  debt  to  Democritus  was  by  far  the 
largest.  While  he  wrote  many  treatises,  only  a  few  frag- 
ments have  been  saved.  He  was  peculiarly  fortunate, 
however,  in  having  a  disciple,  T.  Lucretius  Carus,  who, 
some  250  years  after  his  death,  with  far  more  facile  pen 
than  the  master  and  more  pleasing  style,  recorded  and 
defended  his  system  and  transmitted  it  to  posterity.  In 
his  great  poem,  De  Natura  Rerum,  Lucretius  has  care- 
fully reproduced  the  Epicurean  beliefs  as  to  natural 
science. 

It  is,  of  course,  beyond  the  purpose  here  to  discuss  this 
system  of  philosophy  in  any  other  regard  save  as  it 
touches  upon  natural  science.  It  is  sufficient  to  say  that 
it  was  thoroughly  materialistic,  endeavoring  in  mechani- 
cal cause  to  find  the  explanation  of  all  things.  Nor  is  it 
necessary  to  repeat  those  atomistic  portions  of  the  system 
which  were  borrowed  from  Democritus.  Bodily  reality 
was  for  him  the  only  form  of  reality.  Corporeal  sub- 
stance was  the  only  kind  of  substance.  Besides  this,  the 
assumption  of  empty  space  was  necessary  to  explain 
phenomena.  All  bodies  of  which  we  are  sensible  are 
made  up  of  parts.  If  they  could  be  divided  infinitely, 
they  would  ultimately  be  resolved  into  the  non-existent. 
The  indivisible  ultimate  components  are  the  primary 

i  Aristotle  :  "Gen.  et  Cor.,"  1, 10 ;   327  b  22  ;  328  a  10. 


28  A   STUDY   OF   THE   ATOMS. 

bodies  which,  differing  in  size,  shape,  and  weight,  have  no 
empty  spaces  in  themselves.  They  are  too  small  to  im- 
press themselves  upon  the  senses,  still  they  are  not  mathe- 
matical points.1  All  material  things  are  composed  of 
these  atoms  and  voids  or  empty  spaces.  Epicurus  en- 
deavored to  meet  the  objection  of  Aristotle  to  the  theory 
of  the  downward  motion  of  atoms,  namely,  there  could 
be  no  up  and  down  in  space,  by  appealing  to  experience, 
something  always  appearing  above  our  heads  and  others 
beneath  our  feet.2 

His  most  important  deviation  from  Democritus  was  in 
his  denying  that  the  perpendicular  fall  of  the  atoms  could 
bring  about  a  meeting  and  so  cause  the  rotary  motion 
held  by  the  latter  as  essential  for  world  building.  Ac- 
cording to  Epicurus,  all  atoms  would  fall  equally  fast  in 
empty  space,  and  a  meeting  to  produce  the  rotary  motion 
would  be  impossible  if  they  fell  perpendicularly,3  a  bit  of 
reasoning  borrowed  from  Aristotle.  It  was  necessary  to 
assume  a  slight  swerving  aside  from  the  perpendicular 
in  falling,  to  bring  about  such  a  meeting.  And  this  was 
further  a  necessary  assumption  in  order  to  account  for 
freedom  of  the  will  in  animals. 
Again, 

If  all  motion  in  a  chain  were  bound 
If  new  from  old  in  fixed  order  flowed, 
Cause'linked  to  cause  in  an  eternal  round. 
If  atoms  no  concealed  clinamen  had, 
Cause  to  create  and  break  the  bond  of  fate, 
How  could  free  will  in  animals  exist  ?* 

This  declination    (clinamen)  from  the    straight   line 
sprang  from  the  self  motion  of  the  atoms.    Thus  meeting 

1  Lucretius,  I,  266. 

2  Diogenes  Laertius,  60. 

»  Lucretius  :  "De  Natura  Rerum,"  II,  225. 
4  Lucretius  :  Book  II,  251. 


ANCIENT  VIEWS.  29 

became  possible  and  so  all  the  sequences  of  re-bounding, 
rotary  motion,  clustering  of  atoms  and  world-building. 

This  atomic  declination  is  the  most  original  part  of  the 
philosophy  of  Epicurus  and  is  spoken  of  as  "the  central 
and  truly  original  point  of  the  Epicurean  system."1  It 
was  necessary  as  his  reasoning  brought  him  to  the 
dilemma  of  choice  between  the  creative  design  of  the  older 
philosophers  and  the  fate  or  necessity  of  the  Stoics, 
neither  of  which  satisfied  him.  This  theory  gives  to  the 
atom  of  the  senseless  stone  the  same  self-motion  or  spon- 
taneity or  will  that  was  supposed  to  exist  in  the  atoms  of 
the  human  body,  and  not  merely  does  this  reside  in  the 
individual  atom  but  in  the  mass  of  stone.2  It  is  not  to 
be  understood,  however,  that  Epicurus  endowed  his 
atoms  with  life.  It  was  with  will  only,  and  it  is  difficult 
to  decide  whether  Epicurus  limited  this  declination  to 
the  origin  in  the  case  of  inanimate  matter  and  continued 
it  in  force  for  all  endowed  with  life  and  equally  difficult 
to  see  how  he  reconciled  it  with  the  idea  of  unchanging 
atoms  and  fixed,  constant  law  or  necessity,  a  principle 
very  strongly  insisted  upon  by  him  and  his  followers.8 

g  Thus  two  distinct  schools  of  thought  were 

founded  among  the  Greek  philosophers.  The 
Peripatetics,  followers  of  Aristotle,  looked  upon  matter  as 
continuous  and  filling  all  space,  and  denied  the  existence 
of  indivisible  particles  or  void  spaces.  On  the  other  hand, 
the  Epicureans,  or  atomists,  adopted  the  theories  of 
Democritus  as  modified  by  Epicurus  and  maintained  that 
matter  does  not  fill  all  space  and  is  not  infinitely  divisible 
but  that  it  is  built  up  of  atoms  or  particles  which  cannot 
be  further  divided.  It  was  not  possible  for  a  final  de- 

1  Guyan  :  "I^a  Morale  d'Epicure,"  and  cd.,  p.  99,  note. 
8  Masson  :  "Atomic  Theory  of  Lucretius, "  219. 
3  Masson  :  "Atomic  Theory  of  I^ucretius,"  221. 


30  A   STUDY   OF  THE   ATOMS. 

cision  to  be  reached  between  these  two  views  since  all 
direct  proof  was  lacking.  These  intellectual  giants  had 
reached  the  limits  to  which  it  was  possible  for  the  ob- 
servations and  appliances  at  their  command  to  lead  them. 
No  one  can  thoughtfully  study  the  works  of  these 
philosophers  without  paying  tribute  to  the  intellectual 
acumen  and  the  masterly  logic  which  enabled  them  to 
reason  so  clearly  upon  matters  so  difficult  to  comprehend 
as  the  primal  elements  and  the  nature  of  all  things. 
Especially  is  admiration  aroused  when  one  considers  the 
imperfections  of  their  actual  knowledge  and  the  almost 
total  absence  of  means  for  increasing  knowledge  and  cor- 
recting erroneous  observations. 

It   may    be    questioned   whether   the 

®  Greeks    were    a    race  possessing   the 

as  UDservers.  . 

qualities  or  mind  necessary  for  great 

advance  in  practical  science,  which  comes  only  through 
patient  drudgery,  the  slow  amassing  of  observations,  and 
painstaking  accuracy  as  to  details.  Yet  their  unequaled 
masterpieces  of  sculpture  and  fidelity  to  the  details  of 
anatomy  would  indicate  the  possession  of  great  powers  of 
observation,  of  imitation,  and  of  perfection  of  mechanical 
skill. 

The  Greek  philosopher  possessed,  however,  only  the 
crudest  methods  of  observation,  not  to  be  compared  with 
the  wealth  of  means  at  the  service  of  the  modern  man 
of  science.  He  had  the  very  difficult  task  of  constructing 
the  beliefs  and  defining  the  elementary  physical  concep- 
tion when  the  unaided  eye  determined  the  limit  of  the 
research  and  the  empirical  processes  were  few  and  unre- 
liable. His  rule  and  compasses  and  a  few  makeshifts 
constituted  his  stock  of  apparatus.  It  would  have  been 
miraculous  if  he  had  not  made  mistakes,  it  was  almost  a 


ANCIENT  VIEWS.  31 

miracle,  certainly  a  great  triumph  of  reason,  that  he  saw 
as  far  and  as  clearly  as  he  did. 

In  the  matter  of  observation  and  classification  the 
Greeks  seem  to  have  reached  a  high  plane  of  excellence. 
Taking  Aristotle  as  the  highest  type,  we  find  in  his 
Natural  History  such  careful  observation  of  species  and 
variations  of  habits  of  animals  that  his  work  can  serve  as 
a  foundation  in  zoological  researches  of  the  present  day, 
and  although  in  his  division  of  animals  into  the  blooded 
and  bloodless  he  made  use  of  a  faulty  generalization,  his 
accurate  observation  of  resemblances  and  differences  en- 
abled him  to  separate  properly  the  great  classes  of  verte- 
brates and  invertebrates.  The  estimate  of  Aristotle  given 
by  Tyndall1  is  probably  a  fair  picture  of  the  failings  of 
the  Greek  philosophers  as  men  of  science.  He  finds  in 
his  ideas  indefiniteness,  a  confused  understanding,  too 
great  reliance  upon  the  use  of  language  which  leads  to 
the  self-deception  that  he  was  the  master  of  a  great  sub- 
ject when  he  had  not  even  succeeded  in  grasping  the  ele- 
ments of  it.  He  put  words  in  the  place  of  things,  subject 
in  the  place  of  object.  He  preached  induction  without 
practicing  it  in  that  he  reversed  the  proper  order  of  re- 
search by  proceeding  from  the  general  to  the  special  in- 
stead of  from  the  special  to  the  general. 

The  Greeks  seem  to  have  made  little  true  use  of  induc- 
tive logic  but  it  would  seem  that  their  chief  failure  lay  in 
the  neglect  of  experiment.  Perhaps  the  most  striking 
proof  of  this  is  the  fact  that  they  devised  no  instruments 
nor  apparatus  of  importance  to  aid  them  in  their  observa- 
tion. He  who  experiments,  of  very  necessity  exercises  all  of 
his  ingenuity  and  mechanical  skill  to  devise  contrivances 
which  will  aid  him  in  reaching  his  cherished  goal. 

1  Tyndall :  "Religion  and  Science,"  Brit.  Assoc.  Adv.  of  Science,  1873. 


32  A   STUDY   OF   THE   ATOMS. 

The   sequence  of   methods  which   in   the 

JJ0?*  hands  of  the  modern  man  of  science  has 

Methods. 

enabled  him  to  achieve  such  success  in  the 

study  of  nature  is  as  follows :  There  must  first  be  obser- 
vation and  then  a  logical  classification  of  the  facts  or  phe- 
nomena observed.  By  deductive  logic  the  causal  rela- 
tions are  sought  out  and  found  ;  by  inductive  logic  the 
underlying  law  is  reached.  Each  step  is  tested  and 
proved  by  all  conceivable  experimentation,  much  of  it  of 
the  most  ingenious  description.  These  are  the  tools 
placed  in  his  hands  by  the  ages. 

The   Greeks  used   with   masterly    skill 

th* ^Greeks  their  one  tool)  deductive  lo&ic>  butit  was 
powerless  to  lead  them  to  correct  con- 
clusions when  the  observations  were  faulty  and  the 
touchstone  of  experiment  was  not  applied.  Perhaps  no 
single  sentence  can  better  explain  their  failure  than  the 
following  taken  from  a  letter  of  Epicurus  to  Herodotus  i1 
"For  we  have  still  greater  need  of  a  correct  notion  of  the 
whole,  than  we  have  of  an  accurate  understanding  of  the 
details."  The  first  step  in  natural  knowledge  as  laid 
down  by  Epicurus,1  namely,  that  from  appearances  we 
must  advance  to  their  hidden  causes,  from  the  known  to 
the  unknown,  is  correct  in  principle  but  was  poorly  fol- 
lowed by  him  and  his  pupils.  Furthermore,  how  could 
accuracy  of  observation  be  expected  when  it  was  laid 
down  as  a  principle  that  what  immediately  affects  our 
senses  is  not  the  object  itself,  but  a  picture  of  the  object 
and  these  pictures  may  be  innumerable,  a  different  one 
being  the  cause  of  each  sensation.  Though  these  pic- 
tures, emanating  from  the  same  object,  may  be  nearly 

1  Diogenes:  Laertius  Epic,,  24, 
*  Diogenes:  Laertius  Epic.,  33. 


ANCIENT  VIEWS.  33 

alike,  it  is  possible  that  they  may  differ.  If  the  same 
object  appears  different  to  different  observers,  it  is  be- 
cause different  pictures  must  have  affected  their  senses. 
It  is  not  our  senses  that  are  at  fault,  then,  in  case  of 
mistakes,  but  our  judgment  in  that  it  draws  from  pic- 
tures unwarranted  inferences  as  to  their  cause.1 

Others  of  the  philosophers  would  divest  themselves 
altogether  of  observation  or  sensation  and  trust  to  logic 
alone  as  the  means  of  acquiring  knowledge  and  finding 
out  the  truth. 

In  this  connection  the  famous  arguments 

°f  Zen°  a£ainst  the  possibility  of  motion 
may  well  be  repeated  here  as  exhibiting 

the  character  of  the  logic  by  which  these  philosophers 

reached  their  conclusions.2 

1.  Before  the  body  that  is  moved  can  arrive  at  the 
goal,    it  must  first  have  arrived  at  the  middle    of  the 
course  ;  before  it  reaches  this  point,  it  must  have  arrived 
at  the  middle  of  the  first  half,  and  previously  to  that  at 
the  middle   of  the  first   quarter   and   so   ad  infinitum. 
Every  body,  therefore,  in  order  to  attain  to  one   point 
from  another  must  pass  through  infinitely  many  spaces. 
But  the   infinite  cannot  be  passed  through  in  a  given 
time.       It  is  consequently  impossible  to  arrive  at    one 
point  from  another,  and  motion  is  impossible. 

2.  This  is  the  so-called  Achilles  argument.     The  slow- 
est creature,  the  tortoise,   could  never  be  overtaken  by 
the   swiftest,   Achilles,    if  it  had  once  made  a  step  in 
advance  of  him.     For,  in  order  to  overtake  the  tortoise, 
Achilles  must  first  reach  the  point  where  the  tortoise  was 
when  he  started ;  next  the  point  to  which  it  had  pro- 

1  Zeller:  Stoics,  Epicureans  and  Skeptics,  431. 
Zeller  :  Pre-Socratic  Philosophy,  620  et  seq.  Aristotle  Phys.,  6,  9. 


34  A  STUDY  OF  THE  ATOMS. 

gressed  in  the  interval,  then  the  point  which  it  attained 
while  he  made  this  second  advance,  and  so  on  adinfinitum, 
But  if  it  be  impossible  that  the  slower  should  be  over- 
taken by  the  swifter,  it  is,  generally  speaking,  impossible, 
to  reach  a  given  end  and  motion  is  impossible. 

3.  So  long  as  anything  remains  in  one  and  the  same 
space,  it  is  at  rest.     But  the  flying  arrow  is  at  every 
moment  in  the  same  space.       It  rests,  therefore,  at  every 
moment  of  its  flight,   therefore  its  motion  during   the 
whole  course  is  only  apparent. 

4.  The  fourth  argument  refers  to  the  relation  of  the 
time  of  movement  to  the  space  which  has  to  be  traversed. 
According  to  the  laws  of  motion,  spaces  of  equal  size 
must  be  traversed  in  equal  time  if  the  speed  is  equal. 
But  two  bodies  of  equal  size  move  past  one  another  twice 
as  fast,  if  they  are  both  moving  at  equal  speed,  as  if  one 
of  them  is  still  and  the  other  with  the  same  motion  passes 
by  it.     Hence  Zeno  concludes  that  in  order  to  traverse 
the  same  space  —  the  space  taken  up  by  each  of  these  two 
bodies  —  at  the  same  speed,  only  half  the  time  is  neces- 
sary in  the  one  case  that  is  necessary  in  the  other.     Con- 
sequently,   facts  here  contradict   the  laws   of    motion. 
These  arguments,  it  may  be  added,  were  picked  to  pieces 
by  Aristotle  and  it  is  not  necessary  to  comment  upon 
them  here. 

Sifting  the  chaff  from  the  wheat, 
the  *ain  in  distinct  ideas  as  to  the 


nature  of  matter  may  be  summed 
up  as  follows  :  The  idea  of  elemental  substances  had  been 
grasped  —  not  just  such  elements  as  the  chemist  of  to-day 
knows  —  yet  elements  out  of  which  all  things  were  made, 
the  principles  of  things,  elements  it  is  true  with  inter- 
changeable properties  and  capable  of  transmutation.  The 


ANCIENT  VIEWS.  35 

existence  of  atoms  had  been  well  thought  out.  They 
differed  in  weight  and  form  and  magnitude  ;  they  were 
in  incessant  motion ;  compounds  were  formed  by  their 
union  and  motion  conferred  upon  their  compounds. 

No  place  of  rest  is  found 
To  primal  bodies  through  the  vast  profound, 
And  finding  none,  they  cease  not  ceaseless  rounds. 
Part  forced  together,  wide  asunder  leap : 
From  closer  blow  part,  grappling  with  their  kind, 
In  close  affinities  unite  and  form 
Bodies  of  various  figure — varied  form  diverse.1 

Again : 

For  infinite  atoms  in  a  boundless  void, 

By  endless  motions  build  the  frame  of  things.2 

All  things  are  made  up  of  these  atoms.  In  their  com- 
pounds they  do  not  touch  but  are  separated  by  void 
spaces.  These  atoms  were  not  subject  to  wear  and  could 
not  be  destroyed.  Therefore,  long  before  the  time  of 
Lavoisier  or  of  Maquenne,  matter  was  declared  indestructi- 
ble. 

Nature  reserving  these  as  seeds  of  things 

Permits  in  them  no  minish  nor  decay  ; 

They  can't  be  fewer  and  they  can't  be  less.1 

Referring  to  compounds  Lucretius  writes  : 
Decay  of  some  leaves  others  free  to  grow 
And  thus  the  sum  of  things  rests  unimpaired.* 

The  store  of  elements  material 
Admits  no  diminution,  no  increase.5 

Among  other  views  of  the  Greeks  which  did  not  fall 
far  short  of  the  truth  as  it  is  held  at  present  are  some  of 
the  surmises  as  to  chemical  affinity.  Further,  the  un- 

1 Lucretius :  Trans,  by  Johnson,  Book  I.,  80. 
2  Lucretius  :  Book  I,  63. 
«  I^ucrctius  :  Book  I,  57. 
4  Lucretius  :  Book  II,  79. 
•  Lucretius  :  Book  II,  86. 


36  A   STUDY   OF  THE   ATOMS. 

changeable  nature  of  natural  law  was  recognized,  though 
this  did  not  prevent  a  belief  in  the  most  infinite  mutabil- 
ity and  variability  of  natural  phenomena.  The  existence 
of  ether,  or  the  quinta  essentia,  was  and  is  still  assumed  as 
a  necessity  for  the  explanation  of  various  phenomena. 
Lastly  the  great  thought  of  the  harmony  and  essential 
unity  of  nature  was  dreamed  of  as  it  is  dreamed  of  to-day. 


CHAPTER  II. 


From     the    Greek    Philosophers     to 
Dalton. 


CHAPTER  II. 
FROM  THE  GREEK  PHILOSOPHERS  TO  DALTON. 

It  is  as  if  one  stepped  from  the  glow  of  a  well-lighted 
room  into  the  darkness  of  the  night,  to  pass  from  the 
culture  and  brilliancy  of  the  Greek  schools  to  the  cen- 
turies which  followed  their  decay.  For  many  gener- 
ations there  were  no  new  theories  and  speculations  con- 
cerning the  constitution  of  matter  or  the  nature  of  the 
universe,  but  only  imitations  and  repetitions  of  the  logic 
and  thought  of  the  great  masters.  Among  these  teachers 
who  were  imitated  towered  Aristotle,  and  gradually  he  so 
dominated  philosophy  as  to  be  the  unquestioned  author- 
ity to  whom  all  appeal  was  to  be  made.  Such  conditions 
tended  to  decadence  rather  than  to  progress. 

A  closer  examination  will  show  that  this  state  of  affairs 
was  rather  to  be  expected.  All  that  could  be  learned  by 
deductive  logic  had  been  gleaned  so  far  as  it  was  of  value. 
The  greatest  height  attainable  by  this  means  alone  had 
been  reached.  This  the  Greeks  had  accomplished  in 
little  more  than  two  centuries.  The  refining  and  polish- 
ing of  this  material  yielded  nothing  new,  nor  could  it 
add  to  the  stability  of  the  foundation.  The  theorizing 
had  gone  far  beyond  evidence,  and  something  else  was 
needed  to  settle  the  great  questions  which  had  been 
raised.  It  is  not  strange  then  that  further  efforts  along 
this  line  produced  no  master  spirits  to  take  the  places  of 
the  giants  lost. 

Century  after  century  seemed  to 
Development  of  Ex-         H  [h         l      { 

pen  mental  Science.  _  J  .  6. 

trace  of  progress.     But  this  view 

is  found  to  be  scarcely   true   when   tested   by   another 


40  A   STUDY   OF   THE    ATOMS. 

standard.  The  slow  development  of  experimental 
science,  so  largely  neglected  by  the  brilliant  thinkers 
of  Greece,  was  taking  place.  Many  things  had  con- 
spired to  make  this  difficult.  The  fact  that  it  had  been 
ignored  by  such  men  fostered  prejudice  against  the 
work.  Knowledge  of  nature,  they  had  said,  was  to  be 
gained  by  introspection  and  logic  (microcosm)  rather  than 
by  observation  of  external  phenomena  (macrocosm). 
Material  experiments  were  left  to  quacks  and  charlatans: 
to  those  who  sought  to  deceive  others  rather  than  to  find 
out  the  truth:  to  those  who  would  learn  the  secrets  of 
nature  for  their  own  enriching  or  for  wonder-working. 
The  pathway  upward  was  a  long  and  dark  one.  Instru- 
ments must  be  provided  to  magnify  the  range  of  the 
senses  and  multiply  man's  powers.  Apparatus  must  be 
devised  and  methods  of  research  worked  out.  There 
could  be  little  community  of  work  in  all  of  this,  for  the 
workers  suspected  and  often  hated  each  other;  little 
clear  and  direct  transmission  of  knowledge,  for  the 
ignorant  and  envious  persecuted  any  who  laid  claim  to 
knowledge  beyond  the  common  ken.  It  is,  of  course,  to 
be  questioned  whether  there  would  have  been  any  work- 
ers or  progress  without  the  attraction  of  the  chimeras 
followed,  such  as  the  transmutation  of  base  metals  into 
gold,  the  philosopher's  stone  and  the  elixir  of  life.  The 
pursuit  of  a  vision  or  a  superstition  has  led  men  to  many 
of  their  greatest  discoveries  and  bravest  achievements. 
It  does  not  seem  probable  that  in  the  first  centuries  after 
the  birth  of  Christianity  many  would  have  sought  for 
truth  if  the  reward  consisted  solely  in  its  discovery. 

There  was  much  of  superstition  and  mysticism  among 
these  workers.  The  old  Greek  idea  of  an  overruling 
Necessity  or  Fate  largely  influenced  them.  On  the  walls 


GREEK   PHILOSOPHERS   TO   D  ALTON.  41 

of  their  laboratories  was  inscribed  the  legend,  ' AvayKrj, 
and  the  ' '  Ora,  Labora' '  placed  on  other  walls  meant  a  servile 
effort  at  appeasing  a  god  who  had  the  power  and  might 
have  the  will  to  nullify  all  of  their  labors.  There  have 
been  others  who  have  bowed  down  to  this  fetish  Necessity 
in  later  times,  but  Huxley  has  well  stated  the  position  of 
the  true  man  of  science  :  ' '  Fact  I  know ;  and  Law  I  know ; 
but  what  is  this  Necessity  save  an  empty  shadow  of  my 
own  mind's  throwing."1 

As  for  the  training  of  young  and  enthusiastic  scientific 
workers,  such  a  thing  was  not  dreamed  of  except  in  so  far 
as  it  was  necessary  to  initiate  some  favored  apprentice, 
and  most  of  the  work  was  done  in  secret.  Indeed  it 
was  not  until  a  Liebig  arose  and  the  second  quarter 
of  the  i  Qth  century  had  come  that  laboratories  were 
thrown  open  to  any  and  all  who  chose  to  take  advantage 
of  them,  and  Liebig  met  with  jeering  and  opposition  in 
working  this  great  reform.2  Until  that  time,  special  in- 
fluence was  necessary  to  secure  for  an  ambitious  young 
man  the  opportunity  to  devote  his  energies  to  such 
work. 

The  struggle  upwards  then  to  the  light  seems  drearily 
slow.  It  took  centuries  for  a  telescope  and  microscope  to 
be  invented  and  a  spirit  lamp  and  balance  to  be  brought 
into  general  use  in  chemistry.  Generation  followed  gen- 
eration before  a  Keppler  was  born  to  discover  the  laws  that 
govern  the  movements  of  the  planets  and  the  harmony  of 
the  universe ;  or  a  Torricelli  to  devise  the  experiment  which 
should  settle,  in  part,  the  old  dispute  as  to  the  existence  of 
a  vacuum.  It  was  more  than  1900  years  from  Aristotle 
to  Paracelsus  who  should  cast  off  the  dwarfing  bondage 
to  authority  which  made  all  science  but  slavish  imitation. 

i  Huxley  :  "  Physical  Basis  of  I,ife." 

1  Roth,  "Justus  von  t,iebig  :  Sammlung  Chetn.  Vort.,"  HI,  166. 


42  A   STUDY   OF  THE   ATOMS. 

A  Galileo  was  needed  to  correct  the  erroneous  views  as  to 
the  sun  and  the  earth  and  give  release  from  some  of  the 
ridiculous  theories  of  the  ancients,  and  a  Linnaeus  to  re- 
store and  improve  the  system  of  Natural  History.  After 
nearly  two  millenia  a  Bacon  and  then  Comte  added  the 
last  of  the  needed  tools  for  man's  equipment,  namely, 
inductive  philosophy,  though  this  is  but  the  logic  of  com- 
mon sense,  as  Huxley  says.  Aided  thus  by  the  accumu- 
lated knowledge  and  discoveries  of  many  centuries,  it  be- 
came possible  for  one  equipped  with  even  moderate  men- 
tal capacity  to  make  great  advancement,  and  for  a  Newton 
to  read  deep  in  the  book  of  Nature. 

It  must  be  borne  in  mind  that  during 
Eclipse  of  much  of  the  earlier  p0rtion  of  the  dark 

Knowledge.  %i  v          r 

ages,  there  was  actual  loss  or  eclipse  of 

knowledge.  Superstition  and  ignorance  replaced  the 
better  understanding  of  the  ancients  in  many  cases  of  in- 
terpretation of  natural  phenomena.  Thus  Hoefer  gives1 
certain  examples  to  show  this  retrogression.  All  the  world 
at  present  knows  of  the  accidents  caused  in  mines  by 
asphyxiation.  The  ancients  explained  this  as  due  to  the 
existence  of  irrespirableairs,  which  they  said  extinguished 
the  lamps  of  the  miners  at  the  same  time  that  they  de- 
stroyed life.  The  alchemists,  however,  did  not  speak 
of  irrespirable  airs,  but  of  malignant  demons  who,  wish- 
ing to  put  a  stop  to  the  work  in  the  mines,  treacherously 
slew  the  miners.  Again,  as  to  the  cause  of  the  ascent  of 
water  in  a  pump,  Vitruvius  said  it  was  due  to  the  air, 
though  he  failed  to  give  any  demonstration  of  its  work- 
ing. The  physicists  of  the  middle  ages  ascribed  it  to 
nature's  abhorrence  of  a  vacuum,  which  was  also  one  of 
the  ancient  theories. 

1  Hoefer  :  "  Histoire  de  la  Chitnie,"  2. 


GREEK   PHILOSOPHERS  TO   DAI/TON.  43 

Returning  to  the  study  of  the  atom,  it  will 
be  found  that  the  Practical  workers,  the 
Grecian  alchemists  and  those  of  the  middle 
ages  did  not  so  much  as  take  into  consideration  the 
atomic  theory.  Berthollet  states1  that  the  word  'atom*  is 
not  to  be  found  in  the  Greek  alchemical  manuscripts  ex- 
cept in  one  or  two  doubtful  passages.  The  alchemists, 
by  unvarying  tradition  and  expressed  theories,  attached 
themselves  to  the  doctrines  of  the  Pythagorean  school  as 
taught  by  Plato  in  Timaeus,  and  this  was  true  down  to 
the  close  of  the  i8th  century.  A  passage  in  the  compila- 
tion of  ancient  wisdom  given  by  Isidorus  of  Seville  (636 
A.D.)  would  seem  to  indicate  a  degeneration  of  the  atom 
into  the  ultimate,  or  indivisible  unit  of  various  classes  of 
things.  Thus  he  says  :2  ' '  There  are  then  atoms  in  bodies, 
in  time,  in  number  or  in  words" — meaning  in  these  latter 
cases  the  minute,  the  number  one,  and  the  letters  with 
which  words  are  written.  Stephanus3  (620  A.  D. ) ,  whose 
works  consist  of  nine  lessons  addressed  to  Emperor  Hera- 
clius  and  a  letter  to  Theodorus,  writes  in  Lesson  VI  of 
the  indivisible  atoms  which  constitute  all  bodies.  Only 
a  few  scattered  references  of  this  character  are  to  be 
found  in  the  literature  of  the  early  centuries  of  the  era. 

The  domination  of  the  church  militated 
Opposition  of  against  speculations  as  to  the  origin 
tne  Cnurcn.  . 

and  nature  of  the  universe,   fearing 

that  the  mechanical,  or  indeed  any  general  explanation 
of  the  phenomena  of  nature,  would  remove  the  necessity 
for  a  divine  creator  and  so  would  eliminate  God  from  the 
universe.  Thus  Thomas  Aquinas  considered  all  such 
striving  a  sin,  except  in  so  far  as  it  was  directed  toward 

1  Berthollet :  "  I,es  Origines  de  PAlchimie,"  p.  263. 

2  Isidorus  :  "Orig.  de  Mundo,"  Ub.  XIII.    Cap.  II. 
Berthollet :  "  Chimie  des  Anciens,"  p.  289. 


44  A   STUDY   OF   THE   ATOMS. 

a  better  knowledge  of  God.  The  study  of  nature  was  at 
one  time  largely  turned  over  to  those  who  would  use  it 
in  their  profession  of  healing  and  thus  physicus  became 
the  synonym  of  medicus  and  from  this  came  the  English 
word  physician.  In  the  writings  of  the  fathers  of  the 
early  Christian  Church,  a  favorite  object  of  attack  was 
the  atomic  theory  of  the  ancients  and  contumely  was 
heaped  upon  it.  Lasswitz1  says  that  they  could  not  re- 
peat often  enough  that  busying  oneself  with  physics,  as 
the  Grecian  philosophers  had  done,  was  not  only  a  vain 
exertion  which  could  only  turn  upon  the  unnecessary  and 
useless,  and  in  its  object  was  far  beyond  the  measure  and 
strength  of  the  human  mental  grasp,  but  that  it  included 
a  danger  for  the  safety  of  the  soul  as  the  examples  of 
Leucippus  and  Democritus  proved  who  were  led  away 
into  atheism.  The  views  of  the  Atomistic  school  were 
represented  as  being  very  absurd  and  the  atomists  them- 
selves as  blind  and  pitiable  creatures.  This  mockery  of 
the  theory  was  particularly  directed  at  the  supposed 
motion  of  the  atoms,  their  meeting,  combination,  and 
thus  the  formation  of  the  universe.  Of  course  the  point 
of  offense  was,  as  has  been  stated,  the  materialistic  view 
of  nature,  the  elimination  of  a  designing  power  in  creation 
and  the  ascription  of  the  formation  of  the  universe  to  the 
fortuitous  concourse  of  atoms.  This  attack  was  first 
from  the  side  of  the  defenders  of  the  ancient  deities  and 
was  taken  up  more  vigorously  and  successfully  by  the 
Christian  Church.  Many  of  these  writers  contented 
themselves  with  abuse  and  ridicule.  Thus  Dionysius 
Alexandrinus2  wrote :  ' '  Ye  blind ;  do  then  the  atoms 
bring  you  snow  and  rain  so  that  the  earth  and  all  living 
nature  may  bear  nourishment  for  you  ?  Why  then  do 

1  Lasswitz:  "Geschichte  der  Atomistik,"  i,  13. 
8  I^asswitz,  I,  16. 


GREEK   PHILOSOPHERS   TO   DALTON.  45 

you  not  fall  down  before  your  atoms  and  offer  them  sacri- 
fices as  to  the  lords  of  the  harvest  ?  Ye  ungrateful  ones, 
not  once  from  the  many  gifts  which  ye  have  received 
have  ye  offered  them  the  first  fruits."  Eusebius,  in  his 
Preparatio  evangelica,  quotes  with  approval  this  tract  of 
Dionysius,  not  troubling  himself  over  its  lack  of  argu- 
ment. Lactantius,1  in  the  Fourth  Century,  did  make  a 
crude  attempt  at  a  scientific  refutation  of  the  Atomistic 
doctrine.  "  Who  has  seen,  felt  or  heard  these  atoms?" 
he  asked.  He  saw  in  the  diversity  of  nature  an  argu- 
ment against  the  formation  of  the  universe  out  of  par- 
ticles. Again,  if  the  atoms  were  light  and  spherical  they 
could  in  no  case  hold  firmly  to  one  another  so  as  to  form 
a  corporeal  substance.  If  rough  or  hooked  so  as  to  hold 
on  to  one  another  then  they  must  be  divisible  into  parts. 
Do  water  and  fire  also  consist  of  atoms  as  maintained  by 
Lucretius?  Then  how  is  it  that  fire  is  kindled  even  in 
the  deepest  cold,  when  a  glass  globe  filled  with  water  is 
held  between  the  sun  and  tinder  ?  Were  the  ' '  seeds  of 
fire  "  in  the  water?  Certainly  they  were  not  in  the  sun, 
for  that  cannot  set  tinder  on  fire  even  in  midsummer. 
As  to  animals,  if  you  grant  that  limbs  and  bones  and 
nerves  and  blood  are  made  up  of  atoms,  what  about  per- 
ception, thought,  memory,  spirit?  By  the  bringing  to- 
gether of  what  "seeds"  can  they  be  formed?  "By  the 
finest,"  says  Lucretius.  Then  there  must  be  coarser, 
argues  Lactantius,  and  in  that  he  sees  an  admission  of 
divisibility.  His  chief  argument  is  directed  against  the 
possibility  of  a  mechanical,  accidental  meeting  of  sense- 
less things,  and  thus  the  production  of  the  beautiful 
harmony  and  adaptative  of  the  universe,  without  any 
supervising  or  directive  agency. 

This  style  of  argument  was  much  better  than  the  ridi- 

1  Iyactantius  :  "  De  ira  Dei  ad  Donatum."  I,sw. 


46  A   STUDY  OF  THE  ATOMS. 

cule,  abuse,  and  misrepresentation  which  has  been  ad- 
verted to.  The  theory  of  the  atomists  was  in  truth  only 
what  would  be  called  in  these  days  a  working  hypothesis. 
The  verdict  concerning  it  must  unquestionably  have  been 
' '  not  proven, ' '  and  the  arguments  of  Aristotle  had  really 
placed  it  in  a  very  questionable  light.  It  offered,  how- 
ever, apparently  such  a  simple  means  of  explaining  diffi- 
cult problems  that  its  advocates  had  pushed  its  use  and 
interpretation  to  extremes  which  rendered  them  exceed- 
ingly vulnerable.  But  ridicule  is  easier  than  finding  out 
the  defects  in  an  opponent's  arguments,  and  besides  it 
requires  no  learning  and  is  after  all  more  effective  than 
argument  with  the  ignorant  masses. 

Augustine  writes1  :  "  It  had  been  better  had  I  never 
heard  the  name  of  Democritus  than  that  I  should  think 
with  pain  that  once  a  man  was  considered  great,  by  those 
of  his  time,  who  believed  the  gods  were  pictures  which 
flowed  from  fixed  bodies  without  being  themselves  fixed. ' ' 
He  cites  the  arguments  of  Cicero  against  the  Epicureans 
and  his  expositions  coincide  with  those  of  the  great 
Roman.  They  complement  also  the  arguments  of  I,ac- 
tantius  in  attacking  the  perception  and  recognition  theory 
of  the  atomists,  among  other  things  asking  very  shrewdly 
how,  granting  the  existence  of  atoms  and  the  consequent 
claims  of  the  atomists,  is  it  possible  for  atoms  to  think 
atoms  or  in  any  way  to  become  cognizant  of  them  ? 

Of  course  these  old-world  disputations  have  little  inter- 
est now  and  it  has  been  necessary  to  go  into  them  so  far, 
only  to  make  clear  one  of  the  reasons  why  men  cared 
little  to  take  up  this  theory,  either  to  seek  to  confirm  it 
or  to  use  it  in  explaining  the  varied  problems  presented 
by  nature.  The  all-dominating  Church  frowned  upon 

i  Augustine  •  "  Epistola  ad  Dioscorutn"  Op.  Tom.,  II,  248.  I^sw. 


GREEK   PHILOSOPHERS  TO   DAI/TON.  47 

many  such  inquiries.  At  the  conclusion  of  his  argument 
mentioned  above,  Augustine  apologized  for  "  touching 
such  filth.  Why  should  the  Christian  trouble  himself  to 
find  any  outward  explanation  of  the  marvels  of  nature? 
Leave  that  to  the  heathen."  Going  still  farther,  in  1245 
the  Dominican  order  forbade  the  study  of  physics.  So 
far  as  philosophy  was  cultivated  it  was  that  of  the  Neo- 
Platonic  school  which  offered  little  for  the  scientific  an- 
swering of  questions  as  to  the  nature  of  things  but  the 
age  asked  few  questions  and  cared  little  for  scientific  an- 
swers. 

In  the  seventh  and  eighth  centuries  we  have 
Kinds  <  onjy  a  strav  reference  or  two  to  atoms  and 

most  of  these  have  already  been  cited.  This 
theory  of  the  ancients  had  almost  passed  from  memory 
and  the  word  had  received  new  meaning.  It  has  been 
mentioned  ho wlsidorus,  of  Seville,  whose  writings  formed 
the  thesaurus  of  culture  and  knowledge  of  his  times,  dis- 
tinguished between  atoms  of  bodies,  of  time,  of  number, 
and  of  written  language.  Here  the  word  meant  the  ulti- 
mate unit,  distinct  and  indivisible.  But  he  also  recog- 
nized the  ancient  use  of  the  word.  "  The  philosophers 
call  atoms  certain  particles  of  bodies  so  exceedingly  small 
that  they  cannot  be  seen  nor  can  they  be  divided.  They 
are  said  to  fly  in  restless  motion  through  the  void  of  the 
entire  world  and  are  borne  hither  and  thither  like  the 
dust  in  the  sunbeams  so  that  out  of  them  all  trees,  vege- 
tables and  fruits  spring,  also  fire,  water,  and  everything 
come  from  them  and  consist  of  them  according  to  the 
belief  of  certain  of  the  heathen."  This  passage  and  the 
distinction  between  the  varieties  of  atoms  are  quoted  by 
Venerable  Bede.1  He  divides  the  hour  as  follows  : 

1  Venerable  Bede,  Op.,  I,  90.    I^sw. 


48  A    STUDY   OF   THE   ATOMS. 

f  4  puncti  soils  a  2  ^A  minuta  \ 

i  hora  =  <  /   .  V    —   10   mi- 

(  5  puncti  lunse  a  2  minuta       ) 

nuta  =  40  momenta  =  22,560  atomi. 
The  word  atom  entered  more  and  more  into  common 
speech  to  signify  anything  very  small  and  not  farther 
divisible.  The  musician,  the  astrologer  measured  by 
atoms  ;  the  grammarian  spoke  of  them,  and  in  general 
the  word  denoted  a  moment,  a  sand  grain,  a  particle 
of  dust,  etc.  The  word  had  lost  its  metaphysical 
meaning  and  the  philosophical  theory  was  no  more 
thought  of.1 

It  would  be  going  too  far  afield  to  attempt 
enera  tQ  f0^ow  tjje  rjse  an(j  prOgress  of  philosoph- 

ical discussions  and  schools  during  the 
middle  ages.  While  these  bore  upon  the  question  of  the 
nature  of  matter  and  the  universe,  and  have  their  value 
from  the  standpoint  of  the  philosopher,  they  brought  no 
confirmation  or  refutation  of  the  doctrine  of  atoms  nor  did 
they  advance  the  knowledge  of  nature  and  so  they  may 
well  be  omitted  here.  As  to  general  theories  the  four 
elements  of  Empedocles  were  generally  accepted  as  the 
components  of  all  things,  and  their  nature  was  discussed. 
The  principle  of  the  indestructibility  of  matter,  so  clearly 
stated  by  Parmenides  and  Epicurus,  was  reiterated  by 
some  though  apparently  forgotten  by  others.  Thus 
Adelard,  of  Bath,2  said  :  "  Nothing  is  ever  entirely  de- 
stroyed. When  a  combination  of  particles  with  others 
ceases,  their  existence  does  not  cease  but  they  go  into 
another  combination. ' ' 

In  the  "  Elementa  Philosophise"  of  William  of  Conches, 
there  seems  to  be  a  very  careful  avoidance  of  the  word 

1  I*asswitz  :  "  Gesch.  d.  Atomistik,"  I,  90. 

2  Adelard  of  Bath,  Translated  by  Stahr,  quoted  by  I^asswitz,  I  p.  71. 


GREEK   PHILOSOPHERS  TO   DAI/TON.  49 

atom  and  yet  the  idea  is  well  preserved.  All  bodies  con- 
sist of  elements.  By  an  element  one  must  understand  the 
simplest  and  smallest  particles  of  a  body.  These  elemen- 
tary particles  (particula}  are  invisible  and  indivisible 
except  in  thought.  He  made  use  of  the  word  homiomera, 
showing  his  knowledge  of  the  writings  of  Aristotle  and 
the  source  of  his  ideas  as  to  matter.  He  stated  further 
that  the  elements  were  not  properties  but  matter.  Proper- 
ties reside  in  the  elements  but  are  not  the  elements 
themselves.  The  elements  are  rather  simple  par- 
ticles which  determine  the  properties  of  bodies  by 
their  coming  together.  These  are  the  prima  principia. 
They  were  first  created  and  then  out  of  them  all  other 
things.1 

These  and  a  few  other  references  during 
Influence  of  these  earlier  centuries  up  to  the  i2th  are 
Aristotle. 

the  only  evidences  of  any  effort  to  keep 

alive  the  doctrine  of  atoms.  With  the  introduction  of 
Arabic  learning  into  Europe  came  the  wisdom  of  the 
ancients,  which  they  had  preserved,  and  the  chief  source 
from  which  they  drew  their  learning  was  Aristotle.  The 
practical  disappearance  of  the  atomic  hypothesis  may  be 
attributed  to  his  influence.  He  was  the  arch-antagonist 
of  the  atomists  and  with  the  predominance  of  his  phil- 
osophy over  schools,  backed  by  the  opposition  of  the 
Church  to  atoms  and  everything  else  that  smacked  of 
materialism,  the  atomic  hypothesis  practically  disappeared 
from  view  for  several  centuries,  despite  growth  in  mathe- 
matical and  physical  knowledge  which  should  have  sup- 
ported it.  The  opposition  of  Cicero  and  Seneca  and 
especially  of  Galen  among  the  ancient  authorities  cannot 
have  failed  also  to  have  had  great  weight. 

1  Wil.  dc  Conchis  :  "  Elem.  pbil.,"  p.  209.     I,sw. 


50  A   STUDY   OP  THE   ATOMS. 

The  study  of  Aristotle  by  the  Arabians  did 
Arabian  not  entireiy  prevent  their  including  the 

idea  of  atoms  in  their  philosophy,  yet  it 
led  to  certain  modifications  of  the  doctrine  so  as  to 
attempt  to  meet  the  objections  of  Aristotle.  Their  atoms 
were  without  magnitude  yet  having  position.  By  the 
definite  position  to  one  another  form  was  given  and  the 
power  of  occupying  space.  It  must  be  remembered  that 
the  Arabians  were  especially  noted  for  their  cultivation 
of  the  mathematical  sciences.  Now  in  their  theory  all 
of  their  atoms  were  alike,  their  number  being  changeable 
at  the  will  of  the  Creator.  In  order  that  they  might 
move  there  must  be  vacua,  otherwise  if  all  space  were 
full  of  atoms  some  would  penetrate  others  in  moving. 
Each  atom  was  inseparable  from  certain  conditions  as 
smell,  color,  motion  or  rest.  Magnitude,  however,  is 
not  a  condition  but  belongs  to  compound  bodies  only. 
L,if  e  and  perception  were  among  the  conditions  insepa- 
rable from  the  atoms  and  here  we  have  the  origin  of  the 
hylozoic  views  of  the  alchemists  and  early  chemists. 
There  was  difference  of  opinion  as  to  whether  the  atoms 
were  gifted  with  thought,  knowledge  and  souls.  These 
views  are  instructive  chiefly  as  showing  a  transition  state 
of  the  atomic  doctrine  and  the  effort  so  to  modify  and 
mold  it  as  to  accord  with  the  accepted  views  of  their  re- 
ligion, and  to  disarm  antagonism.  The  scholiasts,  who 
followed  the  Arabians  and  Arabists,  chiefly  engaged  in 
word-splitting  and  in  the  setting-up  of  arbitrary  ideals 
which  led  away  from  nature.  "It  is  better  to  dig  into 
nature,"  says  Bacon,  "than  to  build  upon  your  abstract 
ideas.  It  was  the  analysis  of  nature  which  occupied  the 
school  of  Democritus  and  so  it  penetrated  deeper  than 
others  into  nature." 


GREEK    PHILOSOPHERS   TO   DAI/TON.  51 

As    mathematical    knowledge    grew, 

S°me  interest  was  sllown  in  attempting 
to  prove  the  Aristotelian  view  that 
matter  was  continuous.  The  particles  of  matter  were 
usually  regarded  as  mathematical  points.  Roger  Bacon 
sought  to  solve  the  problem  by  a  mathematical  search  for 
a  body  of  regular  form  which  could  fill  space  without 
leaving  any  vacant  spaces.  He  believed  this  was  pos- 
sible for  hexahedra,  tetrahedra,  and  octahedra.  This 
would  conform  with  the  Platonic  hypothesis  so  far  as  the 
cubical  earth  particles,  the  octahedral  air  and  the  tetra- 
hedral  fire  were  concerned,  but  not  as  to  the  others  and 
after  all  this  is  nothing  more  than  an  expression  on  the 
part  of  Bacon  of  the  belief  common  to  all  the  alchemists. 
Bacon's  being,  however,  only  a  partial  acceptance  of  the 
Platonic  doctrines  and  not  excluding  the  possibility  of  a 
vacuum,  differed  from  the  views  of  all  the  scholiasts  who 
were  agreed  as  to  the  impossibility  of  a  vacuum. 

Aristotle  had  indicated  his 
Conditions  of  Ele-  beljef  ^  tfae  elements  when 

ments  in  Compounds. 

they  unite  to  form  com- 
pounds, though  suffering  change  of  properties,  did  not 
cease  to  exist.  He  left  it  to  be  decided  whether  they  ex- 
isted actually  or  potentially.  This  point  was  taken  up 
and  discussed  with  zest  by  the  Arabians.  Ibn  Sina  con- 
tended for  the  actual  existence,  the  persistence  of  the 
unchanged  form.  Averrhoes  thought  that  the  form  of 
the  elements  must  also  be  changed.  If  the  compound 
derived  its  properties  through  the  loss  of  those  properties 
to  the  elements,  then  it  could  have  no  substantial  form 
unless  those  of  the  elements  were  changed.  The  influ- 
ence of  the  Pythagorean  ideas  is  easily  to  be  traced  in 
this  argument.  Albertus  Magnus  adopted  the  hypothesis 


52  A   STUDY   OF  THE   ATOMS. 

of  Avicenna,  that  an  element  has  a  double  existence.  In 
the  first  state,  when  free,  it  possessed  all  of  its  natural 
characteristics;  in  the  second,  or  bound  (ligatum)  state 
it  is  influenced  by  other  elements.  Hence,  in  compounds, 
the  element,  although  bound,  is  the  same  element,  though 
only  in  potentiality.  This  was  rejected  as  an  explanation 
by  his  pupil,  Thomas  Aquinas,  and  his  opinion  as  that  of 
the  Angelic  Doctor  prevailed.  His  view  seems  to  have 
been  that  the  influence  of  the  elements  upon  one  another 
resulted  in  properties  which  are  the  means  of  the  others 
and  the  forms  are  included  in  these  properties.  Duns 
Scotus  (1308)  maintained  that  the  elements  lost  their 
existence  when  they  entered  into  combination,  but  in 
that  act  they  took  on  a  higher  existence.  The  combina- 
tion also  did  not  come  about  by  the  self -interact  ion  of  the 
elements,  but  was  brought  about  by  some  general  and 
natural  agency,  thus  opposing  the  view  of  a  life  or  soul 
in  the  particles  themselves.  From  the  i4th  century  on, 
the  number  of  adherents  to  the  doctrine  of  the  persistence 
of  the  elements  increased,  adopting  either  the  view  of  Al- 
bertus  Magnus  or  of  Averrhoes. 

Passing  by  the  indeterminate 

Van  Helmont  and  the  discussions  of  the  next  three 
Corpuscular  Theory. 

centuries,  there  is  reached  in 

the  teachings  of  Van  Helmont  (1577-1644)  a  transition 
to  the  corpuscular  theory.  His  first  great  service  con- 
sisted in  his  maintaining  the  existence  of  two  primal,  un- 
changeable and  non-transmutable  elements,  water  and  air.1 
But  it  is  from  water  that  he  believed  most  substances  to 
have  been  formed,  and  it  is  not  perfectly  clear  always 
as  to  the  part  played  by  air  in  his  theory.  His  most  im- 
portant contribution  to  the  corpuscular  theory  is  in  his 

1  Van  Helmont :  "  Causie  et  Initia,"  33,  p.  29 ;  l,asswitz,  I,  344. 


GREEK   PHILOSOPHERS   TO   DAI/TON.  53 

representation  of  what  takes  place  in  the  changes  of  water 
into  steam,  and  in  the  distinction  drawn  by  him  between 
vapor  and  gas.  In  this  latter  case,  he  says  the  difference 
lies  in  the  arrangement  of  the  fundamental  substances  in 
their  smallest  particles.  This  is  very  crude,  however,  and 
has  only  a  far-off  resemblance  to  the  allotropism  or  isomer- 
ism  of  the  present  day.  Van  Helmont  makes  frequent 
mention  of  the  motion  of  atoms,  but  by  this  term  meant 
merely  very  small  particles  without  any  reference  to  their 
indivisibility. 

Giordano  From  a  purely  metaphysical  standpoint, 
Bruno,  Bruno  did  much  to  pave  the  way  for  the 

154  -  DO.  resuscitation  of  the  corpuscular  theory. 
Matter  was  to  him  no  longer  the  passive  substratum  of 
all  nature,  as  imagined  by  Aristotle,  but  all  possible 
things  at  once,  embracing  in  itself  all  forms  and  dimen- 
sions. Matter  was  a  unit,  in  eternal  motion,  one  and  in- 
separable with  force  in  harmonic  order  and  in  organic  and 
necessary  development.  The  search  after  unity  was  a 
necessary  condition  of  all  knowledge.  There  must  be  in 
all  things  an  ultimate,  smallest,  indivisible  unit,  a  mini- 
mum, of  which  all  things  consisted.  This  was  not  merely 
the  physically  smallest  and  indivisible  of  space,  but  the 
absolute,  simple,  and  unchanging.  This  minimum  Bruno 
also  called  ' '  monad, ' '  a  word  which  originally  meant  the 
unit  of  numbers,  but  which  became  later  a  favorite  term 
in  metaphysics.  The  corporeal  minimum  is  the  atom  or 
primordial  body.  For  this  he  decided  the  only  possible 
form  was  the  spherical. 

The  hypothesis  of  spiritual  matter,  a  quinta  essentia, 
or  subtle  stuff,  which  was  not  properly  body  nor  yet  spirit, 
since  in  the  one  case  it  could  not  be  perceived  by  the 
senses,  in  the  other  case  it  occupied  space  ;  a  something 


54  A   STUDY   OF  THE   ATOMS. 

then  which  occupied  an  intermediate  position  between 
corporeal  and  spiritual  matter,  an  ether  or  a  spirit ;  such 
an  hypothesis  was  wide-spread  among  the  ancient  philoso- 
phers and  was  accepted  by  Aristotle.  This  ether  Bruno 
identified  with  the  vacuum  of  Democritus.  It  filled  all 
space  between  bodies  and  between  the  spherical  atoms.  In 
this,  bodies  could  move  without  restriction.  It  was  the 
bearer  of  all  force.  It  was  the  world-soul,  the  dynamic 
of  nature.  Herein  was  it  different  from  the  modern  con- 
ception of  ether  as  simply  a  mechanical  medium.  It  must 
be  borne  in  mind  that  Bruno  was  not  a  physicist  but  a 
poet,  and  his  view  of  the  universe  was  largely  poetical. 
That  he  looked  upon  solid  bodies  as  consisting  of  atoms 
did  not  spring  from  a  physical  necessity  for  explaining 
phenomena,  but  was  merely  the  outcome  of  his  metaphys- 
ical doctrine  of  monads,  to  which  we  are  indeed  indebted 
for  a  number  of  fundamental  conceptions,  but  rather  in 
the  realm  of  philosophy  than  of  natural  science.1 

In  Lubin  the  corpuscular  theory  had  a  note- 
"»  worthy  defender  who  maintained  the  log- 

ical necessity  for  the  conception  of  atoms 
and  endeavored  to  meet  all  the  objections  urged  by  Aris- 
totle and  the  Scholiasts.  The  basis  of  his  arguments  lay 
in  the  impossibility  of  conceiving  the  infinite.  Since  con- 
tinued division  could  have  no  end  and  was,  therefore,  in- 
finite and  inconceivable,  all  substances  in  nature  must 
consist  of  indivisible  atoms. 

Francis  Bacon,  in  his  "Cogitationes  de 

Natura  Rerum> '  did  much to  strengthen 
the  tendency  toward  a  return  to  the 
atomic  hypothesis  as  a  necessity  for  the  explanation 
of  natural  phenomena,  and  as  a  basis  for  physical  science. 

1 1,asswitz  :  "  Gcsch.  d.  Atomistik,"  I,  396. 


GREEK   PHILOSOPHERS  TO   DAI/TON.  55 

His  work  was  in  the  main  mathematical  and  metaphysical. 
In  the  first  place  he  restated  the  dogma  of  the  constancy 
of  matter.  Nothing  could  come  into  being  out  of  nothing, 
and  something  could  not  pass  away  into  nothing.  The 
atomic  idea  might  be  grasped  in  either  of  two  ways.  An 
atom  might  be  conceived  as  the  utmost  bound  of  the  di- 
vision of  bodies,  or  secondly  as  a  body  which  contained 
no  empty  space.  It  is  manifest  that  division  can  go  far 
beyond  the  detection  of  sight  or  of  sense,  for  odors  are 
invisible,  yet  must  consist  of  particles  of  the  bodies,  for 
they  can  be  rubbed  or  washed  from  articles  to  which  they 
have  fastened.  In  his  Novum  Organum  he  seemed  like 
Leibnitz,  to  have  gone  over  from  the  atomistic  view  to 
that  of  matter  as  an  elastic  fluid.  He  no  longer  spoke  of 
a  limit  of  divisibility  and  left  the  question  of  empty  space 
undecided. 

In  Daniel  Sennert  we  have  the  first 
f3  "iei16Sennert>  man,  trained  to  experimental  science, 

who  arose  as  a  defender  of  the  atomic 
hypothesis  since  the  time  of  Democritus.  He  was  pro- 
fessor of  medicine  at  Wittenberg  and  one  of  the  most 
skilful  chemists  of  his  day.  The  principle  upon  which 
he  worked  was  that  the  observation  of  the  whole  alone 
was  no  aid  to  progress.  One  must  descend  to  the  details 
and  observe  closely  nature  itself.  For  the  theoretical 
explanation  he  made  use  of  the  theory  of  atoms.  As 
proofs  that  bodies  consist  of  aggregations  of  atoms  he 
adduced  the  formation  of  smoke  by  burning  bodies  and 
the  process  of  sublimation.  This  latter  was  regarded 
also  as  a  proof  that  the  fine  particles  did  not  change  in 
nature.  Again,  solutions  of  substances,  as  in  mineral 
springs,  may  be  perfectly  clear  and  transparent,  yet  in- 
crustations form  from  the  separating  out  of  very  minute 


56  A   STUDY   OF  THE   ATOMS. 

particles  which  must  have  been  suspended  in  the  liquid 
yet  invisible.  Solution  of  metals  in  acid  or  salts  in 
water  must  then  be  due  to  a  division  of  the  substance 
into  atoms.  Changes  in  natural  substances  are  an  ex- 
change of  outward  form  while  the  particles  remain  the 
same  and  unchanged.1'  For  him  it  was  a  necessary  con- 
clusion, he  wrote  later,  that  there  should  be  certain 
simple  bodies  out  of  which  compound  bodies  were 
formed,  and  into  which  they  could  be  again  re- 
solved. These  simple  bodies  were  physical,  not  mathe- 
matical minima.  He  gave  the  various  names  for  them  : 
minima  naturae,  atomi,  atoma  corpuscula,  acp^ara 
adiaipera,  corpora  indivisibilia.  These  are  the  ultimate 
subdivisions  beyond  which  nature  cannot  go  and  again 
are  the  beginnings  of  all  substances  in  nature.  He 
further  distinguished  between  the  atoms  of  the  elements 
and  atoms  of  compound  bodies.  Thus  there  are  four 
elementary  atoms,  those  of  fire,  air,  water  and  earth. 
The  second  class  were  those  into  which  compounds  were 
divided  in  dissolving  and  mixing,  and  by  their  combina- 
tion new  bodies  were  formed.  The  "  forms"  of  the  atoms, 
which  determine  the  species  of  things,  remain  unchanged. 
Thus,  in  alloying  gold  and  silver  the  atoms  unite  most  in- 
timately but  each  retains  its  distinct  form.  Gold  remains 
gold  and  silver  silver,  as  may  be  seen  by  dissolving  the 
silver  away  with  aqua  fortis,  leaving  the  gold  in  the  form 
of  a  powder.  The  "  form"  of  the  atom  did  not  refer  to 
magnitude,  as  the  atom  possessed  neither  magnitude  nor 
divisibility.  By  the  concourse  of  atoms  the  most  widely 
differing  bodies  could  be  formed.  The  states  of  aggrega- 
tion were  also  explained  by  the  theory  of  atoms.  Clouds 
are  not  continuous  bodies  but  made  up  of  thousands  of 
myriads  of  atoms,  which,  in  forming  rain  and  snow,  again 

1  Sennert:  De  Chymt'a,  XII,  230,  231  (1619),  Lsw. 


GREEK   PHILOSOPHERS   TO   DALTON.  57 

unite.  Condensation  consists  in  the  reuniting  of  atoms 
which  had  been  separated.  So  when  water  evaporated  it 
did  not  change  to  air  but  into  its  own  vapor,  as  mercury 
sent  out  mercury  vapor.  Sennert  had  begun  with  a 
belief  in  the  transmutation  of  the  elements.  It  would 
seem  that  a  change  had  been  wrought  in  his  views  by  the 
study  of  the  atomic  hypothesis.  The  belief  in  the  un- 
changing nature  of  the  elementary  particles  and  the  im- 
possibility of  transmutation  was  growing  and  was  neces- 
sary as  a  foundation  for  all  true  theorizing  in  chemistry. 
As  to  the  cause  of  the  concourse  of  atoms,  Sennert  could 
not  think  of  it  as  fortuitous,  as  was  held  by  Democritus 
and  his  followers,  but  as  being  due  to  the  influence  of  the 
"  forms,"  that  is  to  say,  the  nature.  God  had  so  ordered 
these  forms  that  the  atoms  fitly  arranged  themselves  in 
the  compounds. 

It  is  not  needful  for  our  purpose  to  speak 

Theorists  at  len&th  of  the  theories  of  Gorlaeus 
(1520),  or  of  Basso  (1621),  and  other 
worthy  adherents  of  the  rising  school  of  the  atomists  nor 
can  the  task  of  deciding  their  influence  upon  one  another 
or  upon  Sennert  be  undertaken  here.  Suffice  it  to  say, 
that,  though  misled  in  part  by  erroneous  views,  they  were 
able  and  zealous  in  the  revival  of  the  atomic  philosophy. 
Many  adherents  were  being  won.  In  1624,  in  Paris, 
then  the  center  of  learning,  began  the  agitation  for  ato- 
mistic views  of  nature.  A  public  debate  was  announced 
to  be  held  by  the  defenders  of  these  doctrines,  and  certain 
theses  were  distributed  against  the  views  of  Aristotle  and 
the  Peripatetics.  These  were  condemned  by  the  church 
and  the  punishment  of  the  law  was  threatened  against  all 
who  had  aught  to  do  with  such  doctrines.  Three  of  the 


58  A   STUDY   OF  THE   ATOMS. 

agitators,  De  Claves,  Villon,  and  Bitault,  were  banished 
from  the  city  of  Paris. 

Probably  the  most  potent  factor  in 

Italian  School*  the  renaissance  of  the  doctrine  of 
atoms  and  in  so  modifying  it  as  to 
infuse  new  and  lasting  vitality,  was  the  progress  in  the 
science  of  mechanics  and  physics.  A  new  idea  of  energy 
had  grown  up.  This  idea  of  energy  was  imparted  to  the 
motion  attributed  to  the  atoms.  This  motion,  as  con- 
ceived by  Democritus,  was  merely  a  change  of  place,  and 
though  this  brought  about  a  meeting  of  the  atoms  and  so 
influenced  their  combination,  the  idea  of  an  intense  en- 
ergy resulting  from  the  motion  and  residing  in  the  atoms 
was  lacking.  This  idea  of  energy  is  to  be  seen  in  the 
works  of  Leonardo  da  Vinci,  of  Benedetti  and  of  Galileo. 
It  was  the  office  of  the  latter  to  create,  one  may  almost 
say,  the  new  science  of  physics.  He  considered  motion 
an  original  property  of  unchangeable  matter  and  that  the 
physical  properties  of  this  matter  were  to  be  explained  by 
the  motion  of  the  particles.  Thus,  heat  is  explained  by  him 
as  only  present  in  matter  as  a  motion  of  the  particles. 
The  mere  presence  of  heat  particles  is  insufficient  ;  they 
must  be  in  active  motion.1  A  substance  can  contain  many 
fire  particles  and  yet  be  cold  unless  these  particles  are 
freed  by  motion.  On  account  of  their  fineness  and  great 
velocity  they  can  overcome  the  cohesion  of  particles,  de- 
compose the  body,  or  melt  it,  etc.  Galileo  would  not  ad- 
mit the  possibility  of  a  vacuum.  But  the  most  valuable 
part  of  Galileo's  work  is  the  application  of  experiment  to 
this  problem  of  the  nature  of  matter  and  its  reference  to 
such  mathematical  and  mechanical  principles  as  were 
known  to  him. 

1  Galileo  :  "Op.,"  II,  341,  342.    I,sw. 


GREEK   PHILOSOPHERS   TO   DALTON.  59 

The  atomic  hypothesis  was  now  al- 
most  completely  merged  in  the  cor- 
Theory.  puscular  theory,  in  which  the  ex- 

istence of  particles  was  still  assumed, 
but  these  particles  were  supposed  to  be  indefinitely 
divisible,  and  matter  was  generally  considered  continuous. 
This  has  been  noted  as  the  view  of  Hero,  of  Alexandria, 
and  of  Asclepiades,  of  Bithynia.  It  formed  a  partial 
adaptation  of  the  views  of  Aristotle  and  of  those  of  the 
atomists  without  conceding  the  crux  of  the  atom's  indi- 
visibility. These  views  received  their  highest  develop- 
ment at  the  hands  of  Descartes.  He  was  certainly  one  of 
the  deepest  thinkers  and  most  brilliant  men  of  the  iyth 
century,  and  his  writings  had  great  influence  upon  sub- 
sequent thought  as  well  as  upon  his  contemporaries.  He 
was  thoroughly  trained  in  the  mathematics  and  astronomy 
and  mechanical  physics  of  his  day.  He  wrote  works  on 
physics,  discovered  the  law  of  refraction  of  light,  the  ex- 
planation of  the  rainbow,  knew  well  the  work  of  Keppler 
and  acknowledged  its  influence  upon  him,  and  knew  and 
quoted  Harvey  upon  the  circulation  of  the  blood.  He 
was  the  first  to  suggest  the  explanation  of  the  experiment 
of  Torricelli,  stating  that  the  mercury  was  sustained  by 
the  pressure  of  the  air,  and  suggested  to  Pascal  the  crucial 
test  of  this  explanation  by  making  use  of  the  barometer 
to  measure  the  heights  of  mountains.  The  work  of 
Galileo  and  of  Gassendi  was  known  to  him  and  he  was 
indebted  to  the  theories  of  Sennert,  Gorlaeus  and  especially 
of  Basso.  These  theories  were  announced  in  the  years 
1619-1624.  In  the  latter  year  a  decree  was  made  public 
in  Paris  forbidding  the  promulgation  of  atomistic  or  cor- 
puscular theories  under  pain  of  death,  so  the  theory  of 
Descartes  was  not  made  public  until  after  he  had  left 


60  A  STUDY   OF  THE   ATOMS. 

Paris.  Thus  it  may  be  seen  that  such  a  thinker  as  Des- 
cartes was  prepared  to  make  the  best  use  of  a  very  won- 
derful age  in  which  most  important  discoveries  and  prog- 
ress in  science  were  being  made.  The  method  made  use 
of  by  Descartes  was  the  analytic,  and,  in  contradistinction 
to  the  methods  of  the  ancients,  he  believed  that  a  knowledge 
of  nature  was  to  be  obtained  only  through  impressions 
gained  by  the  senses,  the  chief  of  these  impressions  being 
those  of  extension  and  form. 

The  conclusions  reached  in  his  philosophy  were  that 
there  was  no  vacuum,  that  matter  was  infinitely  divisible 
and  that  there  was  but  one  universe,  infinitely  extended 
yet  composed  of  one  and  the  same  kind  of  matter.  For 
the  idea  of  atoms  there  was  substituted  that  of  small 
particles  or  corpuscles,  for  these  were  needed  in  order  to 
explain  physically  many  phenomena.  These  particles 
were  further  divisible,  yet  were  not  the  secondary  particles 
or  molecules  of  Sennert  and  Basso,  a  theory  of  which  he 
strangely  made  no  use.  It  was  further  supposed  that 
these  particles  were  in  constant  motion  as  were  the  atoms 
of  Democritus,  only  this  idea  of  motion  was  extended. 
A  particle  might  have  many  motions  at  the  same  time,  as 
the  wheel  in  a  watch,  carried  by  a  man  upon  a  ship,  would 
have  its  own  motion,  and  that  of  the  man,  the  ship,  the 
sea  and  the  earth.  Motion  meant  energy  as  well  as  mere 
change  of  place.  There  were  three  elements — fire,  air 
and  earth.  Originally  all  nature  was  filled  with  one 
material,  homogeneous,  fluid,  continuous.  At  creation, 
God  divided  this  into  different  particles  and  it  would  be 
limiting  the  power  of  the  Deity  to  say  that  there  were  in- 
divisible particles  which  he  could  not  subdivide  at  will. 
To  these  particles  he  gave  specific  motions  which  there- 
after distinguished  them  and  gave  the  different  elements. 


GREEK   PHILOSOPHERS   TO   DAI/TON.  6 1 

Thus  there  was  one  original  source  of  the  elements  and 
their  genesis  was  brought  about  by  motion.  The  original 
form  of  the  particles  was  not  of  consequence  ;  in  their 
motion  they  rubbed  together  and  so  lost  edges  and  angles 
and  assumed  such  form  as  was  necessary  to  completely 
fill  space.  They  were  really  conceived  of  at  times  as 
fluid  and  plastic  by  Descartes,  and  at  other  times  as  rigid, 
and  his  system  offers  a  number  of  such  contradictions. 
His  explanation  of  the  striking  of  fire  by  means  of  a  flint 
and  steel  may  serve  to  exemplify  his  manner  of  reasoning. 
The  hard  particles  of  flint  find  themselves  suddenly  sur- 
rounded by  the  ball-like  fire  particles  and  flame  follows.1 
As  to  the  existence  of  a  vacuum,  he  regarded  such  a  thing 
as  unproved  and  its  assumption  as  unnecessary  for  the 
explanation  of  phenomena.  The  supposed  pores  of  bodies 
are  filled  with  particles  of  the  fire  element  which  are  not 
atoms  but  an  extraordinarily  fluid  and  fine  substance. 
In  a  letter  to  Mersenne  (1630)  he  wrote  :  "If  you  now 
grant  me  that  there  is  no  vacant  space,  as  I  think  I  am 
able  to  prove,  then  you  are  forced  to  grant  that  these 
pores  (in  gold,  etc.)  are  full  of  a  matter  which  easily 
penetrates  everywhere." 

It  may  seem  strange  in  these  days  that 
the  views  of  Descartes  as  to  the  exist- 
ence of  empty  space  were  so  little  in- 
fluenced by  the  famous  experiment  of  Torricelli  which 
was  apparently  so  conclusive  on  the  question.  Especially 
might  this  cause  surprise  when  one  thinks  that  Descartes 
was  the  first  to  suggest  that  the  column  of  mercury  was 
upheld  by  the  weight  of  the  atmosphere.  This  experi- 
ment of  Torricelli  with  the  mercury  column  and  the 
empty  space  above  attracted  a  great  deal  of  attention  and 

i  Descartes  :  "  Principia,"  IV,  84,  I^sw. 


62  A  STUDY  OF  THE   ATOMS. 

was  repeated  in  many  places,  arousing  much  interest 
because  of  its  theoretical  bearings.  It  was  modified 
in  various  ways  and  subjected  to  acute  testing  and  reason- 
ing. The  general  opinion  was,  however,  that  it  had  by 
no  means  proved  the  existence  of  an  absolute  vacuum. 
It  really  had  very  little  direct  influence  at  the  time  upon 
atomic  views  because  one  was  not  forced  to  believe  the 
space  above  the  mercury  absolutely  empty,  but  could 
assume  the  presence  of  a  sufficiently  fine  matter.  Still 
many  adhered  to  the  belief  in  a  vacuum.  The  most  im- 
portant contemporary  of  Descartes  retaining  this  view  was 
Gassendi,  who  did  not  base  his  assumption  of  the  vacuum, 
however,  upon  the  Torricellian  experiment  but  upon  the 
necessity  for  a  vacuum  as  an  explanation  of  many  physi- 
cal phenomena.  Thus,  it  was  needed  to  explain  the  ex- 
pansion and  contraction  of  air  and,  in  general,  the  action 
of  heat  and  cold  upon  bodies,  or  again,  to  explain  the 
varying  specific  gravities.  lie  regarded  it  as  an  expla- 
nation of  the  solution  of  a  solid  in  water.  Thus  salt 
particles  are  taken  up  and  held  between  the  particles  of 
water,  i.e. ,  in  the  empty  spaces.  If  this  was  true,  then  when 
water  had  dissolved  all  of  the  salt  it  could  hold,  there 
should  still  be  empty  spaces  in  which  something  else 
could  be  held.  This  view  he  believed  he  confirmed  by 
the  experiment  in  which  he  dissolved  an  amount  of 
alum  in  water  already  saturated  with  salt.  For  his  view 
of  matter,  atoms  were  also  necessary  and  these  were  the 
undecomposable  atoms  of  Democritus. 

Thomas  Hobbes,  who  did  so  much 
H°bbeS'  for  the  advancement  of  physics  and 

for  its  establishment  as  a  science  t 
seemed  to  return  in  part  to  the  methods  of  the  ancients 
in  his  view  as  to  the  best  method  of  discovering  truth. 


GREEK   PHILOSOPHERS   TO   DAI/TON.  63 

In  his  opinion,  knowledge  was  to  be  obtained  with  cer- 
tainty only  by  the  exercise  of  the  reason  and  of  logic  and 
not  from  the  testimony  of  the  senses.  When  closely  ex- 
amined, however,  this  only  meant  that  the  testimony  of 
the  senses  must  be  rigidly  tested  by  the  reason  to  avoid 
error  and  to  advance  truth.  It  was  Thomas  Hobbes  who 
first  maintained  that  geometry  was  the  only  exact  science. 
Physics  was  indebted  to  it  for  all  of  its  true  progress.  A 
science  must  be  based  upon  geometrical  principles.  It 
is  thus  seen  that  he  was  an  important  factor  in  lifting 
science  from  the  level  of  mere  empiricism  and  system- 
atized observations,  and  in  insisting  on  a  proper  basis 
for  theory.  So  far  as  his  theories  bear  upon  the  subject 
under  discussion  they  may  be  briefly  considered.  He 
propounded  in  the  place  of  the  corpuscular  theory  of 
Descartes  that  of  an  original  fluid  matter  with  particles 
readily  slipping  by  and  between  each  other.  He  recog- 
nized no  fixed,  rigid  atoms  or  particles.  There  was  no 
need  for  a  vacuum  and  he  denied  the  existence  of  such 
in  the  barometer.  The  existence  of  an  extremely  fluid 
ether  was  assumed  by  him,  which  had  no  other  motion 
than  that  received  from  the  bodies  moving  in  it. 

It  is  clear  that  it  was  mainly  the  physicists  who  were 
concerned  in  the  reviving  of  the  atomic  views  and  that  it 
was  regarded  as  chiefly  a  physical  problem.  It  was  be- 
cause of  their  efforts  to  explain  physical  phenomena  that 
the  simple  atomic  theory  was  lost  sight  of  for  a  time  and 
that  the  corpuscular  theory,  strange  admixture  of  atoms 
which  were  not  atoms  and  of  the  continuity  hypothesis, 
arose.  Chemists  from  the  time  of  Paracelsus  had  corn- 
batted  the  Aristotelian  doctrines  with  a  theory  of  atoms 
which,  however,  embodied  much  that  was  unscientific 
and  was  imperfectly  formulated  and  only  accepted  here 


64  A  STUDY   OF   THE   ATOMS. 

and  there.  For  them  the  primal  elements  were  ceasing 
to  be  substances  which  could  be  transmuted  the  one  into 
the  other.  They  thought  of  matter  as  possessed  of  a 
living  creative  'force'  such  as  is  seen  in  the  growing 
organisms  of  nature.  They  were  opposed  to  a  mechanical 
conception  of  nature. 

Now,  in  Robert  Boyle  we  have  a  com- 
Rotert  Boyle,  bination  of  chemist  and  physicist  and 

the  highest  type  of  the  experimental 
philosopher  of  his  day.  He  was  most  interested  in  the 
establishment  of  facts  by  experiment,  and  theoretical 
speculations  were  to  him  a  secondary  matter.  Hence  it 
is  that  his  theories  were  not  as  fine  spun  nor  extended  as 
those  of  other  philosophers,  interesting  him  little  beyond 
their  capacity  for  service  in  explanation  of  his  facts. 
There  was  for  him  one  only  and  universal  matter,  com- 
mon to  all  bodies,  extended,  divisible,  and  impenetrable. 
The  differences  in  bodies  sprang  from  the  differences  in 
motion.  The  particles  possessed  magnitude,  form,  and 
motion.  The  order  or  position  of  these  particles  was 
fixed  and  had  to  do  with  the  nature  of  the  body.  There 
were  two  classes  of  particles  ;  the  original  corpuscles,  too 
fine  for  us  to  perceive,  and  stable  groups  of  these  parti- 
cles, hard  to  dissociate,  forming  thus  secondary  particles. 
The  particles  of  the  elements,  earth,  water,  etc.,  were 
themselves  made  up  of  these  fine  particles.  These  could 
further  unite  and  give  the  various  compounds.  All  bod- 
ies, even  those  apparently  solid,  have  pores  and  these  are 
penetrated  and  filled  by  the  effluvia  of  other  bodies. 
These  effluvia  are  breathed  out  by  all  bodies,  and  thus 
every  substance  forms  an  atmosphere  around  it.  To  sup- 
port this  theory  of  the  fine  effluvia  he  adduced  many  facts 
and  experiments  which  are  of  especial  interest  because 


GREEK   PHILOSOPHERS  TO  DALTON.  65 

among  them  the  testimony  of  the  microscope  is  called 
upon  as  an  aid  in  this  discussion,  and  further  the  first 
quantitative  chemical  experiment  is  brought  to  bear  upon 
it.  Ammonia,  he  said,  gives  a  perceptible  blue  in  a  solu- 
tion which  contains  only  the  28,534th  part  of  its  weight 
of  copper  or  the  256,8o6th  part  of  its  volume.  This  he 
looked  upon  as  a  proof  of  the  power  of  copper  to  send  out 
an  exceedingly  minute  effluvium.1 

Boyle  was  especially  desirous  of  giving  a  scientific 
foundation  to  chemistry.  He  hoped  that  chemistry,  lay- 
ing aside  the  aims  of  the  hermetic  art,  would  acquire  a 
new  growth  upwards  and  would  contribute  much,  if  not 
to  the  finding  of  the  elixir,  then  to  the  ennobling  of  the 
human  race  and  the  increase  in  the  knowledge  of  nature.* 
The  best  foundation  for  the  new  chemistry  he  thought 
would  be  the  corpuscular  theory,  and  hence  he  sought  to 
make  this  theory  acceptable  to  chemists.  Boyle  regarded 
those  experiments  in  which  a  body  was  changed  into  one 
compound  and  out  of  this  again  into  its  original  condition 
as  among  the  best  proofs  of  the  truth  of  the  corpuscular 
theory.  Such  experiments  were  inexplicable  from  the 
standpoint  of  the  Aristotelian  theories.  One  of  these 
experiments  which  he  most  highly  regarded  was  the  re- 
production of  niter  out  of  the  constituents  obtained  by  its 
analysis.  Affinity  was  explained  by  him  on  the  mechan- 
ical principles  of  the  corpuscular  theory.  The  corpuscles 
of  sulphur  form  with  those  of  quicksilver  a  stable  com- 
pound, cinnabar,  but  the  corpuscles  of  sal  tartari  (potash) 
unite  yet  more  closely  with  sulphur  so  that  they  set  the 
quicksilver  free  from  the  cinnabar.  A  greater  affinity  was 
to  him,  then,  not  a  question  of  attraction  but  of  the  form  of 
the  particles,  and  was  determined  by  the  possibility  of 


1  Boyle  :  "  Bxerc.  de  mira.  Subtil,  effluv.,"  C.  3,  p.  9. 
8  Boyle  :  "  Spec,  unum  atque  alt."—  Pref. 


66  A   STUDY   OF   THE   ATOMS. 

closer  and  firmer  connection.  It  is  not  possible  here  to 
follow  at  greater  length  Boyle's  application  of  mechanical 
principles  to  the  explanation  of  chemical  phenomena. 
Where  knowledge  of  both  principles  and  phenomena  were 
imperfect,  the  applications  were  of  necessity  faulty.  But 
this  was  a  great  step  in  advance  upon  the  easy  and  mean- 
ingless attributing  of  all  that  was  difficult  to  explain  to 
qualitates  occultae,  souls,  sympathies,  attractions,  etc. 
As  to  the  strife  over  the  existence  of  a  vacuum,  Boyle  de- 
clined to  side  either  with  the  plenists  or  antiplenists.  He 
would  not  assert  that  the  top  of  the  barometer  tube  or  the 
receiver  of  the  air-pump  was  empty,  but  he  said  that  it 
was  certainly  empty  of  air  and  the  elaborate  theory  of 
Hobbes  was  false. 

At  this  time  the  various  attractions  exhibited  in  natural 
phenomena  were  under  consideration,  and  a  number  of 
theories  were  advanced  concerning  them.  As  has  been 
seen,  Boyle  wished  to  substitute  the  property  of  form  and 
the  closeness  of  connection  depending  upon  it  for  the 
elective  affinity  of  chemistry.  Borelli  denied  the  existence 
of  any  attractive  force  or  attraction  in  nature.  His  sub- 
stitute seems  to  have  been  a  propelling  force.  Thus  mag- 
net and  iron  are  by  a  natural  force  set  in  spontaneous 
motion  toward  one  another.  This  could,  of  course,  be  re- 
ferred to  a  primal  force  or  motion. 

Hooke  introduced  the  new  idea  of 

Vibration  Theory  vibration  theOry.  The  conti- 

of  Hooke.  .  .  , , 

nuity  or  matter  was  maintained  by 

him  and  the  filling  of  space  was  looked  upon  as  dependent 
not  merely  upon  the  position  and  size  of  the  particles,  but 
essentially  upon  the  character  of  their  swinging  move- 
ments and  that  all  properties  of  bodies  depended  upon  the 
coincidence  or  interference  of  their  vibrations.  This  he 


GREEK   PHILOSOPHERS   TO   DAI/TON.  67 

called  the  congruity  and  incongruity  of  bodies.  On  the 
hypothesis  of  these  vibrations  he  based  an  undulatory 
theory  of  light,  first  suggested  by  Grimaldi.  An  example 
given  by  him  may  best  illustrate  his  views  as  to  matter. 
Suppose  a  very  thin  plate  of  iron,  one  square  foot  in  area, 
vibrating  backwards  and  forwards  at  right  angles  to  its 
plane  with  such  velocity  that  no  other  body  can  penetrate 
the  space  in  which  it  moves.  If  this  vibration  measures 
one  foot  then  it  has  the  same  effect  as  if  space  were  filled  by 
a  cubic  foot  of  a  body  appreciable  to  the  senses.  In  his 
lecture  on  this  subject  he  said  :  "  I  do  therefore  define  a 
sensible  body  to  be  a  determinate  space  or  extension  de- 
fended from  being  penetrated  by  another  by  a  power  from 
within. 

Huygens  contributed  much  to    the  ad- 
"y S^S*  vancement  of  physics  and  to  the  return  to 

the  atomic  idea.  His  undulatory  theory 
of  light  was  nearly  in  accord  with  the  theory  of  the  present 
'day  but  was  neglected  for  the  sake  of  the  Newtonian 
theory.  His  rotation  theory  as  to  gravity  also  served  to 
show  his  experimental  powers,  clear  insight,  and  acute 
reasoning.  He  assumed  the  existence  of  empty  space  so 
as  to  allow  for  motion.  In  his  theories  he  also  had  need 
of  a  light  ether  and  a  gravitation  ether,  not  fluid  but 
made  up  of  extremely  fine  particles.  It  was  necessary 
too,  that  there  should  be  solid,  indivisible  particles  or 
atoms.  Thus  he  wrote  to  Leibnitz  :  ' '  The  ground  upon 
which  I  am  forced  to  assume  undecomposable  atoms  is 
this,  that  I,  just  as  little  as  you,  can  accommodate  my- 
self to  the  Cartesian  doctrine,  according  to  which  the  ex- 
istence of  a  body  consists  in  its  extension  alone,  and  that 
I  therefore  find  it  necessary  in  order  that  the  bodies  may 


68  A   STUDY   OF   THE   ATOMS. 

retain  their  form  and  resist  opposing  motions,  to  ascribe 
to  them  impenetrability  and  resistance  against  breaking 
and  compressing.  *  *  *  *  The  hypothesis  of  in- 
finite stability  seems  to  me  therefore  very  necessary  and 
I  cannot  understand  why  you  should  find  it  so  strange. ' ' l 
In  considering  the  motion  of  the  atoms  he  introduced  the 
principles  of  mechanics  and  enunciated  the  laws  of 
collision.  In  these  there  was  offered  an  explanation  of 
that  which  had  disconcerted  the  theories  of  Gassendi  and 
Galileo,  and  others  who  adhered  to  the  kinetic  theory  of 
atoms,  namely,  the  reality  of  the  motion  and  its  continuity 
without  loss  from  collisions.  Unchangeable,  non- elastic 
atoms  were  necessary  and  a  transmission  of  the  force 
through  them.  For  the  kinetic  atomists  of  the  lyth  cen- 
tury, then,  all  forces  in  nature,  heat,  light,  electricity, 
gravitation,  chemical  affinity  were  based  upon  the  me- 
chanics of  atomic  motion  and  this  was  the  fundamental 
principle  of  their  natural  philosophy.  A  change  of  one 
force  into  another  was  then  entirely  possible.  The  prob- 
lem remained  to  refer  the  particular  individual  motions  to 
adequate  mathematical  laws.  The  ultimate  motion  of  the 
atoms  could  not  be  reached.  It  was  possible  only  to 
decide  by  comprehensible  mathematical  formulas  the 
observed  motions.  This  was  accomplished  by  Newton  in 
the  laws  of  gravitation. 

Leibnitz  wrote  of  Huygens  :  "Of  all  who  have  main- 
tained the  assumption  of  atoms  none  have  done  so  with 
so  great  knowledge  of  the  causes  nor  have  contributed 
more  to  its  illumination."1  Still  the  theories  of  Huygens 
won  but  few  adherents  because  of  the  opposition  and 
overwhelming  influence  of  Leibnitz  and  of  Newton.  But 
his  thoughts  were  not  lost.  They  exercised  much  influ- 

*  Leibnitz  :  "Math.  Schrift,"  II,  156.    I,sw. 


GREEK   PHILOSOPHERS   TO   DALTON.  69 

ence  over  the  most  thoughtful  men  of  the  time.  The 
development  of  the  calculus  was  a  necessity,  however, 
before  they  could  yield  their  highest  results. 

The  latter  half  of  the  xyth  century  wit- 
Attacks  of  nessed  a  very  determined  onslaught  of 
the  Church. 

theologians     and     churchly     authority 

against  Descartes  and  the  corpuscular  theory.  In  1663 
his  works  were  placed  upon  the  Index  Expurgatorius. 
In  1667  the  erection  of  a  monument  in  Paris  was  forbid- 
den, and  in  the  next  succeeding  years  the  Cartesian  sys- 
tem was  placed  under  the  ban  at  the  most  prominent 
French  universities.  One  of  the  strangest  but  most  po- 
tent arguments  used  by  the  churchmen  was  that  in  the 
light  of  the  corpuscular  theory  the  transubstantiation 
dogma  of  the  Eucharist  became  an  impossibility.  These 
attacks  led  some  atomists  to  endeavor  to  bring  their  theo- 
ries into  harmony  with  the  decrees  of  the  church.  Such 
efforts  had  no  bearing  on  the  development  of  this  theory 
and  have  no  value  nor  interest  here.  It  is  necessary  also 
to  pass  without  mention  the  systems  of  Malebranche  and 
Spinoza. 

Leibnitz,  largely  influenced  by  Hobbes,  in 
1670,  in  his  "Hypothesis Physica,"  main- 
tained the  continuity  of  matter,  the  ex- 
istence of  ether  and  the  motion  of  the  corpuscles.  Matter 
was  fluid  but  he  overcame  the  difficulty  experienced  by 
Descartes  in  introducing  solid  bodies  into  it  by  calling 
those  bodies  rigid  whose  particles  were  in  harmonious 
motion.  The  form  of  a  body  then  was  the  space  occupied 
by  its  moving  particles.  The  penetrating  ether  was  taken 
up  by  these  bodies  in  bubbles  (bulla).  This  theory  of 
bubbles  was  elaborated  very  fully  but  need  not  be  farther 


70  A   STUDY   OF  THE   ATOMS. 

referred  to  here.  The  ether  present  everywhere  in  the 
interstices  of  bodies  was  the  exciting  cause  of  the  motion 
and  of  chemical  reaction.  This  ether  was  then  about  the 
same  as  the  Archaeus  of  Paracelsus  and  Van  Helmont, 
the  Rector  of  Tachenius,  the  Spiritus  Mundi  or  Mercurial 
Principle  of  others.  The  chief  service  done  the  corpus- 
cular theory  by  Leibnitz  consisted  in  his  bringing  mathe- 
matical analysis  to  its  support,  as  Huygens'  consisted  in 
his  applying  the  principle  of  mechanics.  Still  it  was  at 
best  only  one  of  the  possible  hypotheses  suggested  for 
the  explanation  of  natural  phenomena.  Much  more  was 
necessary  in  order  that  it  should  become  the  real  and  only 
explanation,  based  upon  accurate  mathematical  laws  and 
substantiated  by  experiment.  Its  plausibility  won  for  it 
many  adherents,  some  holding  the  true  atomic  view  of 
indivisible  atoms  and  empty  spaces,  others  the  view  of 
a  continuous  matter  and  no  vacuum,  but  all  agreed  upon 
the  motion  of  the  particles  whether  atoms  or  corpuscles. 
Some  of  these  followers  of  the  greatest  thinkers,  pressing 
too  far  in  their  unwise  zeal  that  which  was  at  best  but  a 
plausible  working  hypothesis,  brought  it  into  discredit  and 
compassed  its  downfall  as  the  dominant  philosophical 
theory.  With  the  waning  of  the  corpuscular  theory  the 
hylozoic  theories  took  on  new  life  and  growth.  Matter 
was  again  invested  with  soul  and  life,  and  the  world  be- 
came full  of  the  ghostly  spirits. 

When  one  comes  to  Newton  he  finds  that 
Newton,  there  is  no  effort  at  formulating  a  complete 

theory  on  his  part  as  to  the  constitution  of 
matter,  and  his  thoughts  on  the  subject  are  scattered  here 
and  there  through  his  writings.  The  problem  was  for  the 
time  discredited  or  rather  looked  upon  as  beyond  solution 
with  the  knowledge  and  instruments  at  hand.  He  took  no 


GREEK   PHILOSOPHERS  TO   DAI/TON.  71 

interest  in  speculations  which  gave  promise  of  nothing 
positive,  or,  perhaps  more  truly,  his  interest  was  not 
aroused  in  that  direction.  Yet  he  borrowed  much  from 
the  theories  of  the  philosophers  who  had  preceded  him. 
His  view  of  nature  was  that  of  a  dynamic  rather  than  of 
a  kinetic  atomist.  In  so  far  as  he  assumed  the  existence  of 
rigid  separate  particles  of  matter  his  system  was  based 
upon  the  corpuscular  theory,  but  he  did  not  agree  to  the 
view  that  their  interaction  was  due  solely  to  the  motion 
springing  from  their  meeting  one  another.  In  the  place 
of  the  laws,  which  according  to  Huygens  regulated  this 
imparting  motion  through  collision,  he  substituted  force 
working  at  a  distance.  He  conceived  of  the  ultimate  par- 
ticle as  a  "  solid,  massy,  hard,  impenetrable,  movable 
particle." 

' '  Hypotheses  non  fingo  ' '  was  his  famous  saying,  often 
quoted  as  showing  his  dislike  of  speculations.  In  his 
opinion,  everything  which  does  not  follow  out  of  observa- 
tions as  an  hypothesis  and  hypotheses,  whether  meta- 
physical or  physical,  mechanical  or  those  of  hidden  qual- 
ities, should  not  be  taken  up  in  experimental  physics. 
And  yet,  hypotheses  in  the  hands  of  Huygens  had  made 
possible  the  founding  of  this  very  physics  and  led  him  to  a 
truth  which  the  influence  of  Newton  obscured  for  more  than 
a  century  and  a  half — the  undulatory  theory  of  light.  An 
hypothesis  used  as  an  hypothesis  may  be  most  helpful. 
It  is  dangerous  when  it  comes  to  be  grasped  in  the  place  of 
a  fact  itself,  since  it  was  only  intended  to  explain  facts.  It 
is  interesting  to  note  that  in  spite  of  his  objection  to  hypoth- 
eses Newton  could  not  get  away  from,  or  better,  could  not 
get  along  without,  the  atoms.  As  to  the  ether  hypothesis, 
Newton  wrote  to  Boyle  :  ' '  For  my  part,  I  have  so  little 
taste  for  things  of  this  kind  that  had  not  your  suggestion 


72  A  STUDY  OF  THE   ATOMS. 

led  me  to  it  I  would  never,  I  believe,  have  put  pen  to  paper 
about  it."  He  made  use  of  the  ether  hypothesis  in  his 
paper  before  the  Royal  Society,  entitled  "An  hypothesis 
explaining  the  properties  of  light  "  (1675).  His  theory 
of  gravitation  was  not  regarded  by  him  in  the  light  of  an 
hypothesis.  He  also  made  a  suggestion  as  to  chemical 
force,  which  was  not  classed  by  him  as  an  hypothesis, 
though  it  seems  perilously  near  one.  ' '  I  would  rather 
conclude  from  the  holding  together  of  bodies  that  the  par- 
ticles of  the  same  attract  one  another  with  a  force  which 
in  immediate  contact  is  very  great,  at  a  slight  distance 
have  as  a  consequence  chemical  action,  at  greater  distances 
exercise  no  perceptible  influence."1 

At  the  conclusion  of  his  lecture  course  before  the  Royal 
Institution,  Dalton  transcribed  the  following  extracts  from 
Newton's  "  Principia,"  which,  therefore,  acquire  a  dou- 
ble interest :  ' '  The  parts  of  all  homogenal  hard  bodies, 
which  fully  touch  one  another,  stick  together  very  strongly. 
And  for  explaining  how  this  may  be  some  have  invent- 
ed hooked  atoms,  which  is  begging  the  question ;  and 
others  tell  us  that  bodies  are  glued  together  by  rest,  that 
is,  by  relative  rest  among  themselves.  I  had  rather  infer 
from  their  cohesion  that  their  particles  attract  one  another 
by  some  force,  which  in  immediate  contact  is  exceedingly 
strong,  at  small  distances,  performs  the  chemical  operations 
above  mentioned,  and  reaches  not  far  from  the  particles 
with  any  sensible  effort. 

"All  bodies  seem  to  be  composed  of  hard  particles. 
Even  the  rays  of  light  seem  to  be  hard  bodies,  and  how 
such  very  hard  particles  which  are  only  laid  together  and 
touch  only  in  a  few  points,  can  stick  together,  and  that 
so  firmly  as  they  do,  without  the  assistance  of  something 

1  Newton  :  Op.  IV.  351. 


GREEK   PHILOSOPHERS  TO   DAI/TON.  73 

which  causes  them  to  be  attracted  or  pressed  towards  one 
another,  is  very  difficult  to  conceive. 

"  It  seems  probable  to  me  that  God  in  the  beginning 
formed  matter  in  solid,  massy,  hard,  impenetrable, 
movable  particles  of  such  sizes  and  figures  and  with  such 
other  properties,  and  in  such  proportion  to  space  as  most 
conduced  to  the  end  for  which  He  formed  them  ;  and 
that  these  primitive  particles  being  solids,  are  incom- 
parably harder  than  any  porous  bodies  compounded 
of  them,  even  so  very  hard  as  never  to  wear  or  break  in 
pieces,  no  ordinary  power  being  able  to  divide  what  God 
himself  made  one  in  the  first  creation.  While  the  par- 
ticles continue  entire  they  may  compose  bodies  of  one  and 
the  same  nature  and  texture  in  all  ages ;  but  should  they 
wear  away  or  break  in  pieces,  the  nature  of  things  de- 
pending upon  them  would  be  changed.  Water  and  earth, 
composed  of  old  worn  particles  and  fragments  of  particles, 
would  not  be  of  the  same  nature  and  texture  now,  with 
water  and  earth  composed  of  entire  particles  in  the  begin- 
ning. And,  therefore,  that  nature  may  be  lasting  the 
changes  of  corporeal  things  are  to  be  placed  only  in  the 
various  separations  and  new  associations  and  motions  of 
these  permanent  particles,  compound  bodies  being  apt  to 
break,  not  in  the  midst  of  solid  particles,  but  where  those 
particles  are  laid  together,  and  only  touch  in  a  few  points." 

Again,  "  God  is  able  to  create  particles  of  matter  of 
several  sizes  and  figures,  and  in  several  proportions  to  the 
space  they  occupy,  and  perhaps  of  different  densities  and 
forces.  At  least  I  see  nothing  of  contradiction  in  all  this. ' ' 

Again,  "  Now  by  the  help  of  these  principles  all  mate- 
rial things  seem  to  have  been  composed  of  the  hard  and 
solid  particles  above  mentioned,  variously  associated,  in 
the  first  creation  by  the  counsel  of  an  intelligent  agent." 


74  A   STUDY   OF  THE   ATOMS. 

From  this  time  until  the  close  of  the  1 8th 
oscovic  ,  century,  we  find  the  discussion  of  atoms 
largely  relegated  to  the  mathematicians, 
few  of  them,  even,  caring  to  press  the  investigation  of 
nature  along  this  line.  The  most  important  theorizing 
upon  the  subject  was  done  by  the  Italian  mathematician 
and  natural  philosopher,  Boscovich.  In  his  opinion 
matter  was  made  up  of  atoms,  each  atom  being  an  in- 
divisible point,  having  position  in  space,  capable  of 
motion  in  a  continuous  path  and  possessing  certain  mass. 
It  was  endowed  with  potential  force.  Two  atoms  might 
attract  or  repel  each  other.  Two  atoms  could  never  coin- 
cide, or  occupy  the  same  space  at  the  same  time.  There 
was  no  such  thing  as  actual  contact  between  them,  all 
action  taking  place  at  a  distance.  The  atom  itself  pos- 
sessed no  parts  or  dimensions.  In  its  geometrical  aspect 
it  was  a  mere  geometrical  point,  having  no  extension  in 
space.  Were  this  alone  considered  it  would  be  possible 
for  two  atoms  to  exist  in  the  same  space  but  the  forces 
acting  between  them  prevent  this. 

It  may  be  remarked  that  such  a  view  of  the  atom  is 
mathematically  logical  since  this  is  the  only  kind  of  atom 
which  would  not  be  mathematically  divisible.  The  atom 
of  the  chemist,  having  extension  in  space,  must  be  mathe- 
matically divisible.  Boscovich's  view  of  the  atom  as  a 
geometrical  point  approximates  to  the  modern  view  of 
the  atom  as  a  center  of  forces. 

It  is  not  necessary  to  refer  here  to  the  views  of  that 
other  great  mathematician,  Bernoulli,  although  they  seem 
to  have  influenced  Dal  ton  and  were  quoted  by  him,  at 
least  in  part.  They  contained  no  new  contribution  to  the 
inquiry  we  are  making. 


GREEK  PHILOSOPHERS  TO  DAI/TON.  75 

Following  Newton,  we  find  little  concern 

Neg  ect  o  as  to  the  constitution  of  matter  among 

Hypotheses.  .  .  mi 

the  physicists  and  chemists.     The  old 

hypotheses  were  disregarded  or  forgotten.  Their 
bearing  upon  or  necessity  for  the  development  of  natural 
science  was  not  recognized.  The  new  methods  of  re- 
search and  the  great  impetus  given  to  a  practical  develop- 
ment of  science  by  better  organization  opened  up  fields  of 
such  interest  and  led  to  discoveries  of  such  moment  that 
far-off  theories  were  laid  aside.  It  is  of  little  interest  to 
follow  the  treatment  of  the  theories  by  pure  metaphysi- 
cians as  they  could  do  little  to  develop  them  and  could 
contribute  nothing  to  their  firm  establishment. 

It  is  interesting  to  note  here  the  reproduction  of  the 
condition  of  affairs  which  obtained  after  the  period  of  the 
flowering  of  the  Greek  philosophy.  It  is  as  if  the  race, 
sated  and  wearied  by  a  pursuit  and  refinement  of  theory 
far  beyond  the  evidence  of  facts,  had  turned  for  relief  to 
the  harvesting  of  facts  without  troubling  itself  about 
theory.  Quite  possibly  it  was  not  the  hypotheses  which 
repelled  but  the  mistaking  of  hypotheses  for  facts  and 
their  confident  assertion  as  such.  ' '  Science  warns  us, ' ' 
says  Huxley,  in  his  ' '  Physical  Basis  of  L,ife, "  *  *  that  the 
assertion  which  outstrips  evidence  is  not  only  a  blunder 
but  a  crime. ' '  Bishop  Berkeley  was  very  careful  to  draw 
a  distinction  here  and  if  chemists  accepted  his  view  of  the 
matter  that  may  also  serve  to  explain  why  they  seemed  to 
think  hypotheses  had  little  to  do  with  science.  He 
wrote  :  ' '  What  is  said  of  physical  forces  residing  in  bodies 
whether  attracting  or  repelling,  is  to  be  regarded  only  as 
a  mathematical  hypothesis  and  not  anything  really  exist- 
ing in  nature."1 

1  Berkeley :  "  Sins,"  p.  234. 


CHAPTER  III. 


The  Atomic  Theory  of  Chemistry 


CHAPTER  III. 
THE  ATOMIC  THEORY  OF  CHEMISTRY. 

The  conception  of  atoms  had  up  to  the  close  of  the 
1 8th  century  been  almost  exclusively  the  possession  of 
the  metaphysician  and  the  mathematical  physicist,  and 
had  served  to  develop  their  sciences.  With  the  exception 
of  Sennert  and  Boyle,  chemists  had  contributed  little  to 
its  formulation  and  less  for  its  establishment,  nor  had  they 
derived  inspiration  from  it  for  the  proper  founding  of 
their  own  science.  For  more  than  a  century  they  had 
been  following  the  ignis  fatuus  of  a  false  theory  of  com- 
bustion and  a  most  elusive,  hypothetical  phlogiston. 
The  close  of  the  i8th  century  found  them  engaged  in 
bitter  strife  over  these  theories,  and  too  fully  occupied  to 
think  of  much  else  than  the  wreck  of  the  old  beliefs  and 
the  adaptation  of  the  new.  The  master  mind  of  Lavoisier, 
who  had  wrought  this  revolution,  was  busied  with  the 
greater  work  of  reconstruction  and,  dealing  little  with 
hypotheses  which  could  not  be  directly  proved  by  experi- 
ments in  his  laboratory,  was  laying  broad  and  strong  the 
foundations  of  the  New  Chemistry.  And  so  the  works  of 
Bergman,  Scheele,  Priestley,  Black,  Cavendish,  Macquer 
and  others  do  not  treat  of  atoms  and  their  moving  forces, 
except  in  an  occasional  indefinite  reference  to  some  sort  of 
particle. 

Yet  the  chemist  was  the  very  one  most  needed  to  take 
this',  which  had  been  hitherto  really  but  an  atomic  hy- 
pothesis, and  establish  it  with  all  the  dignity  and  strength 
of  an  atomic  theory.  Up  to  this  time  the  facts  adduced 
to  substantiate  it  had  been  qualitative  only.  To  give  it 
a  quantitative  basis  was  reserved  for  the  igth  century 


8o  A   STUDY   OF   THE   ATOMS. 

and  a  chemist,  and  this  was  the  achievement  of  Dalton. 
It  is  scarcely  possible  to  overestimate  the  service  thus 
rendered  or  to  give  him  too  great  credit  in  connection  with 
the  establishment  of  the  atomic  theory. 

The  justice  of  Dal  ton's  claim  to  the 
Justice  of  title  of  founder  of  the  modern  atomic 

JJalton  s  Claim.  .  ,  ,  L . 

theory  has  been  brought  into  question. 

The  same  idea,  it  has  been  affirmed,  can  be  discovered 
in  the  works  of  Richter  and  perhaps  others.  But  this,  if 
true,  would  only  be  in  accord  with  the  law  that  knowl- 
edge does  not  come  suddenly  but  is  a  growth.  Glimpses 
of  the  light  are  caught  before  the  full  light  of  day  is  re- 
vealed. The  credit  belongs  to  him  who  voices  the  un- 
formulated  and  only  partially  grasped  truth  which  is  in 
men's  minds  and  clearly  states  it  so  as  to  draw  the  atten- 
tion of  all  men  to  it.  Here  and  there  men  had  thought 
out  in  part  the  Periodic  Law,  but  Mendeleeff  will  always 
be  known  as  its  author.  To  Darwin  will  always  be  given 
the  credit  of  the  discovery  of  the  Law  of  Natural  Selec- 
tion and  yet  Wells  and  Mayhew  had  partly  anticipated 
his  Origin  of  Species  by  many  years,  and  he  gives  a  list  of 
thirty-five  others  in  the  early  part  of  the  igth  century 
who  had  faintly  foreshadowed  some  of  his  conclusions. 
And  so  it  rarely  happens  that  one  man  discovers  and  im- 
presses upon  his  age  that  which  is  entirely  new  and  un- 
thought  of.  If  it  is  too  new,  and  too  much  ahead  of 
their  thinking  his  audience  pays  "little  attention  to  it  and 
it  must  wait  until  the  world  grows  wiser  and  broader  and 
can  assimilate  the  thought. 

It  is  a  matter  of  .much  interest  to  know 
what  train  of  thought  led  Dalton  to 
seek  in  the  atomic  theory  an  explana- 
tion of  his  facts,  and  this  point  will  be  discussed  further 


ATOMIC   THEORY   OF   CHEMISTRY.  8 1 

on,  but  it  is  much  more  important  to  understand  upon  what 
he  based  the  revived  hypothesis,  founding  it  so  securely 
that  the  scientific  world  was  at  last  induced  to  accept  the 
hypothesis  and  accord  it  the  position  of  the  great  central 
theory  of  science.  The  three  foundation  stones  made  use 
of  by  Dalton  were  the  quantitative  laws  of  constant  propor- 
tions, of  interproportionality,  and  of  multiple  proportions ; 
three  laws  that  marked  the  true  beginning  of  the  quantita- 
tive period  in  the  science  of  chemistry.  The  first  two  of  these 
laws  were  recognized  by  those  who  preceded  Dalton,  the 
third  he  discovered  and  applied  himself.  While  it  is  true 
that  the  first  two  had  been  recognized  before,  it  is  also  true 
that  the  conception  of  them  was  confused  and  the  enun- 
ciation of  them  far  from  distinct  before  the  atomic  theory 
put  meaning  into  them.  Besides  these  quantitative  facts, 
it  should  be  mentioned  that  the  indestructibility  of  matter, 
the  persistence  of  the  elements  and  the  impossibility  of 
their  transmutation  were  well-recognized  principles. 

In  tracing  the  discovery  of  these 

Lavoisier  and  the        quantitative  laws  it  is  necessary  to 
New  Chemistry. 

have  some  knowledge  of  the  condi- 
tion of  chemical  science  at  the  close  of  the  i8th  century. 
First  and  most  important  for  our  purposes  was  the  intro- 
duction of  a  new  definition  for  the  word  element  which 
enabled  chemists  to  divide  all  bodies  in  nature  into  simple, 
undecomposable  bodies  called  elements,  and  compound 
bodies  made  up  of  those  elements.  Every  substance 
which  could  not  be  decomposed  was  regarded  as  an  ele- 
ment. The  list  of  these  simple  bodies  speedily  became  a 
lengthy  one  and  replaced  the  short  one  of  the  vague 
essences,  or  elements,  of  the  Greeks  and  the  alchemists. 
The  metals  which  had  formerly  been  considered  com- 
pounds containing  the  hypothetical  phlogiston  were  now 


82  A   STUDY  OP   THE   ATOMS. 

recognized  as  simple  and  their  calces  were  known  as  their 
compounds  with  oxygen.  In  these  and  other  compounds, 
chemists  were  busy  determining  the  relations  by  weight 
of  the  constituents.  Many  of  these  analyses  were  carried 
out  with  the  greatest  care  by  the  supporters  of  the  phlo- 
giston theory  in  defense  of  their  beliefs.  As  Kopp  re- 
marks,1 it  is  not  reasonable  to  suppose  that  men  like 
Bergman,  Macquer,  Scheele,  Cavendish  and  others  would 
have  taken  the  trouble  to  make  these  analyses  if  they  had 
not  believed  in  the  constancy  of  proportions  of  the  con- 
stituents they  were  determining,  but  there  is  no  proof  that 
they  reversed  the  thought  and  considered  only  such  as 
showed  constancy  of  proportions  to  be  chemical  com- 
pounds. This  thought  seems  to  have  been  first  grasped 
by  Lavoisier.  That  he  did  grasp  it  is  quite  apparent  from 
his  '  *  Traite  de  Chimie. ' '  The  different  bodies,  as  for  in- 
stance the  different  acids,  are  spoken  of  as  having  defi- 
nite compositions  which  can  be  determined  and  which 
serve  to  distinguish  them.  While  he  wrote  at  first  of  the 
existence  of  an  indefinite  number  of  nitric  acids,  from  the 
colorless  to  the  fuming  and  deepest  colored,  a  few  years 
later  he  taught  that  there  were  three  steps  in  the  com- 
bination of  nitrogen  with  oxygen,  nitric  oxide,  nitrous 
acid,  and  nitric  acid  and  that  the  other  apparently  differ, 
ent  acids  consisted  of  nitric  acid  with  more  or  less  nitric 
oxide  absorbed  in  it.  And  yet  he  did  not  expressly 
state  his  belief  in  the  constancy  of  proportions  nor  lay  it 
down  as  one  of  the  doctrines  of  the  science.  Gradually 
the  importance  of  this  among  the  doctrines  of  chemistry 
came  to  be  recognized,  and  where  it  had  been  tacitly  ac- 
cepted by  many,  Proust  stated  categorically  his  belief 
that  definite  chemical  compounds  contained  fixed  and 
constant  proportions  of  their  constituents  and  supported 

1  Kopp :  "  Entwickclung  der  Chemie,"  p.  221. 


ATOMIC  THEORY  OF  CHEMISTRY.  83 

his  views  by  much  excellent  analytical  work.  This 
statement  was  brought  out  by  the  contention  of  Berthollet 
that  the  proportions  were  not  fixed  but  could  be  indefi- 
nitely varied.  His  views  he  embodied  in  his  famous 
"  Essai  d'un  Statique  Chimique."  The  discussion  be- 
tween these  two  was  not  ended  until  after  Dalton's  an- 
nouncement of  the  atomic  theory.  The  constancy  of 
proportions  was  then  generally  accepted. 

The  idea  of  proportionality  in  the 

w  *  combining  amounts  was  a  matter  of 

proportionality. 

slow  growth  through  the  i8th  cen- 
tury. It  was  difficult  to  detect  any  regularity  or  deduce 
any  reliable  generalization  or  law  because  analytical 
methods  were  imperfect  and  the  analyses  so  faulty  as  to  be 
misleading.  Without  referring  to  the  few  scattered  obser- 
vations which  appeared  previously,  the  first  work  of  im- 
port in  this  line  was  that  of  Bergman  (1775)  followed  by 
Wenzel  and  Kirwan.  The  chief  effort  was  to  determine 
the  amount  of  acid  and  base  respectively  necessary  for  the 
production  of  a  neutral  salt  and  from  that  the  relative 
amounts  of  different  acids  requisite  for  the  neutraliza- 
tion of  any  one  base,  or  the  relative  amounts  of  the  dif- 
ferent bases  for  one  acid.  Bergman  bases  upon  his  anal- 
ysis his  doctrine  of  affinity ,  believing  that  a  base  demanded 
more  acid  for  its  neutralization  the  greater  its  affinity  for 
the  acid.  These  views  were  vigorously  combatted  by 
Berthollet.  Cavendish  in  1767  used  the  term  "equiva- 
lent ' '  to  express  the  amount  of  soda  or  potash  which  cor- 
responded with  a  definite  amount  of  lime  necessary  for 
the  neutralization  of  a  fixed  quantity  of  acid. 

The  most  careful  and  accurate  analytical 
Wenzel.  work  on  this  subject  was  done  by  Wenzel. 

He  published,  in  1777,  his  "  Lehre  von  der 
Verwandtschaft  der  Korper. ' '  Although  he  did  very  ex- 


84  A   STUDY   OF   THE   ATOMS. 

cellent  work  in  this,  which  was  utilized  by  Richter  and 
others  afterwards,  he  failed  to  note  the  crucial  fact, 
namely,  the  persistency  of  the  neutrality  in  the  double 
decomposition  of  the  neutral  salts  and  so  could  not  dis- 
cover the  generalization  which  was  to  be  deduced  from 
this.  On  the  contrary,  he  admitted  that,  the  quantity  of 
the  neutral  salts  which  react  upon  one  another  being 
calculated  from  their  known  composition,  an  excess  of 
one  may  remain  after  the  reaction.  Berzelius  was  there- 
fore in  error  in  speaking  of  Wenzel  as  the  discoverer  of 
the  law,  as  has  been  repeatedly  pointed  out.1 

h  It  is  to  J.  B.   Richter  (1762-1807)   that  the 

credit  of  discovering  the  law  of  interpropor- 
tionality  is  really  to  be  given,  yet  this  fact  was  not  recog- 
nized until  after  the  announcement  of  the  atomic  theory 
and  his  work  received  at  first  but  little  recognition.  This 
was  doubtless  in  part  due  to  erroneous  ideas  which  he 
advanced  along  with  the  true.  Richter' s  earliest  work, 
his  inaugural  dissertation,  showed  the  trend  of  his  mind 
toward  the  application  of  mathematics  to  chemistry.  In 
1792  he  published  his  important  work  on  stoichiometry 
("  Anfangsgriinde  der  Stoichiometrie,  oder  Messkunst 
chem.  Elemente").  In  this  it  is  very  apparent  that  he 
strove  to  establish  numerical  generalizations  as  to  the 
combining  properties  of  acids  and  bases.  He  recognized 
the  permanence  of  neutrality  in  the  double  decomposition 
of  two  neutral  salts  which  Wenzel  had  missed  but  which 
was  also  recognized  by  others  after  Wenzel  had  published 
his  treatise.  From  the  permanence  of  neutrality  he  drew 
the  deduction  that  there  must  be  a  definite  relation  be- 
tween the  masses  of  each  neutral  compound  and  that  the 
terms  of  this  relation  are  of  such  a  nature  that  they  can 

1  Kopp  :  "  Entwickelung  d.  Chemie,"  p.  251. 


ATOMIC   THEORY   OF   CHEMISTRY.  85 

be  determined  from  the  masses  of  the  neutral  compounds. 
Thus  there  is  a  proportionality  between  the  quantities  of 
acids  uniting  with  a  given  weight  of  base  and  between 
the  quantities  of  bases  uniting  with  a  given  weight  of 
acid.  But  Richter  went  further  and  stated  that  these 
quantities  form  a  progression,  the  terms  of  which  bear  to 
each  other  a  simple  ratio,  a  statement  which  is  not  borne 
out  by  the  facts  and  which  consequently  weakened  the 
impression  of  the  former  statement. 

In  1793  he  drew  up  a  table  which  he  called  a  "Series 
of  Masses." 

RICHTER'S  SERIES  OF  MASSES. 

Sulphuric         Muriatic  Nitric 

acid.  acid.  acid. 

Potash i. 606  2.239  1.143 

Soda i. 218  1.699  0.867 

Volatile  alkali 0.638  0.889  0.453 

Baryta 2.224  3.099  1.581 

Lime 0.796  1.107  °-5^5 

Magnesia 0.616  0.858  0.438 

Alumina 0.526  0.734  0.374 

He  showed  how  this  table  might  be  utilized  to  calcu- 
late the  amounts  of  acid  necessary  to  neutralize  known 
amounts  of  bases  and  vice  versa.  As  Wurtz  observes1  the 
forms  of  expression  used  by  him  are  not  clear.  The 
thought  which  he  wished  to  convey  can  be  grasped  in  the 
light  of  later  knowledge,  but  his  unfortunate  choice  of 
terms  and  complicated  statements  must  have  contributed 
to  the  neglect  with  which  his  observations  were  treated. 
Unquestionably  he  was  a  man  of  rare  penetration,  but  it 
is  equally  beyond  doubt  that  it  would  be  most  unjust  to 
credit  him  with  having  anticipated,  in  the  truest  sense,  the 
discovery  of  Dalton.  When  one  considers  that  Richter 
was  still  an  adherent  of  the  phlogistic  doctrines  and  en- 

1  Wurtz  :  "Atomic  Theory,"  p.  16. 


86  A   STUDY   OF   THE   ATOMS. 

deavored  to  reconcile  his  sharp  and  true  insight  into  the 
nature  of  metallic  oxides  (in  which  work  too,  he  extended 
his  observation  upon  neutral  salts  and  showed  that  the 
same  definiteness  of  proportion  and  interproportionality 
was  to  be  observed  in  these  metallic  oxides  as  in  the  neu- 
tral salts) ,  with  this  discredited  theory,  the  appropriateness 
of  Wurtz' s  designation  of  him  as  the  "  profound  but  per- 
plexed author  of  the  law  of  interproportionality  ' '  must 
be  acknowledged. 

Richter  was  much  indebted  to  G.  E.  Fischer  for  the  rec- 
ognition of  his  work.  In  1802,  Fischer  endeavored  to  ex- 
plain and  simplify  his  deductions  and  succeeded  in  making 
the  regularities  much  clearer,  and  thus  aided  in  demon- 
strating the  law  of  proportionality.  Through  Fischer 
the  attention  of  Berthollet  was  drawn  to  Richter's  work, 
and  he  expressed  this  opinion  as  to  its  value  :  "  The  pre- 
ceding observations  seem  to  me  necessarily  to  lead  to  the 
conclusion  that  in  my  researches  I  have  only  hinted  at  the 
laws  of  affinity,  but  that  Richter  has  positively  established 
the  fact  that  the  different  acids  follow  proportions  corre- 
sponding with  the  different  alkaline  bases  in  order  to  pro- 
duce neutrality.  This  fact  may  be  of  the  greatest  utility 
in  verifying  the  experiments  which  have  been  made  upon 
the  proportions  of  the  elements  of  salts,  and  even  to  de- 
termine those  which  have  not  yet  been  decided  by  exper- 
iment, and  so  furnish  the  surest  and  easiest  method  of 
accomplishing  this  object,  so  important  to  chemistry."1 

Fischer  reduced  the  various  series  given  by  Richter  to 
one,  by  giving  the  ratio  which  the  quantities  of  acids  and 
bases  contained  in  the  series  bore  to  one  number,  namely,  to 
looo  parts  of  sulphuric  acid.  This  greatly  simplified  it, 
and,  as  Wurtz  remarks,  is  the  first  table  of  chemical 
equivalents. 

1  Wurtz  :  "Atomic  Theory,"  p.  21. 


ATOMIC  THEORY  OF  CHEMISTRY.  87 

FISCHER'S  TABI,E. 

Alumina 525               Fluoric  acid 427 

Magnesia 415               Carbonic  acid 577 

Ammonia 572               Sebacic  acid 706 

Lime 793               Muriatic  acid 712 

Soda 859               Oxalic  acid 755 

Strontia 1329               Phosphoric  acid . . .  979 

Potash 1605               Formic  acid 988 

Baryta 2222               Sulphuric  acid  ....  1000 

Succinic  acid 1209               Nitric  acid 1405 

Acetic  acid 1480               Citric  acid 1583 

Tartaric  acid 1694 

In  order  to  prepare  a  neutral  salt,  the  requisite  base  and 
acid  must  be  taken  in  the  proportion  of  the  equivalents 
given.  It  may  be  added  that  Richter  had  gone  a  step 
beyond  this  and  had  observed  that  the  amounts  of  differ- 
ent metals  which  combine  with  a  given  weight  of  acid 
will  also  combine  with  a  given  weight  of  oxygen.  It  is 
important  as  bearing  upon  the  claims  of  Dalton  that  but 
little  attention  was  given  to  the  work  of  Richter  until 
eight  or  nine  years  after  it  appeared.  Dalton  states  that 
he  was  ignorant  of  it  until  some  time  after  his  discovery, 
and  Richter  himself  complains  in  1795  that  his  work  was 
looked  upon  by  chemists  as  a  fruitless  speculation. 

The  next  step  from  the  recognition 
Law  of  Multiple  f  uivalents  or  proportionate 

Proportion.  ~*  f.    r 

numbers  was  to  multiple  propor- 
tions, but  to  take  that  step  required  a  clearer  conception 
of  the  meaning  of  the  proportionate  numbers  than  was 
held  by  the  chemists  of  the  time.  While  several  chemists 
seem  to  have  been  so  near  its  discovery  as  only  to  have 
needed  the  enunciation  of  it,  they  failed  to  realize  it  and  to 
state  the  generalization.  In  fact  it  was  not  stated  until 
after  the  conception  of  the  atomic  theory  came  to  Dalton, 
and  it  was  used  by  him  as  one  of  the  facts  which  most 


88  A  STUDY  OF  THE   ATOMS. 

clearly  pointed  to  the  existence  of  atoms  as  its  true  ex- 
planation. 

It  had  for  some  years  been  recognized  that  two  or 
more  compounds  could  be  formed  by  the  same  elements. 
L,avoisier  spoke  of  the  compounds  of  nitrogen  and  oxy- 
gen as  the  two  steps  of  saturation.  Proust,  in  his  dis- 
cussion with  Berthollet,  had  proved  that  such  compounds 
were  definite  in  composition  and  that  the  proportions 
differed  by  leaps,  as  it  were,  and  not  by  continuous  change. 
Richter  had  shown  that  iron  and  mercury  could  combine 
with  oxygen  in  several  proportions  so  as  to  form  several 
oxides.  Furthermore,  it  was  already  customary  to  state 
the  composition  of  compounds  which  contained  the  same 
elements  in  different  proportions  by  giving  one  element 
in  a  fixed  proportion  and  varying  the  proportions  of  the 
second  element.  This  being  the  case  it  would  seem  at 
first  sight  impossible  to  fail  to  detect  the  simple  ratio 
existing  between  the  latter  proportions,  but  the  truth  is, 
the  analyses  were  too  faulty  to  show  this  relation.  Thus 
Proust,  one  of  the  most  careful  and  accurate  chemists  of 
his  day,  stated  that  100  parts  of  copper  combined  with 
i7^i  to  1 8  parts  of  oxygen  to  form  the  red  oxide 
and  with  25  parts  to  form  the  black  oxide.  The  cor- 
rect numbers  are  respectively  12.6  and  25.2.  Richter 
came  nearer  to  the  discovery  of  the  law  than  any  other 
for  he  really  tried  to  derive  numerical  relations  between 
the  different  amounts  of  oxygen  combined  with  the  same 
metal  but  failed  to  prove  the  existence  of  such,  most 
probably  because  of  the  same  imperfect  data.  It  re- 
mained therefore  for  Dalton  to  discover  this  law  and  not 
the  least  of  his  achievements  is  that  he  divined  the  law  by 
some  sort  of  intuition  in  spite  of  faulty  numbers  and  ex- 
periments. His  method  of  reaching  his  discovery  will 
be  discussed  later  in  connection  with  his  atomic  theory. 


ATOMIC  THEORY   OF   CHEMISTRY.  89 

Some  have  maintained  that  Higgins,1 
W.  Higgins.  who  published  in  1790  a  work  dealing 

with  the  conflict  between  the  phlogistic 
and  antiphlogistic  doctrines,  anticipated  Dalton  in  the 
discovery  of  multiple  proportions  and  the  combination  by 
atoms.  An  examination  of  so  much  of  his  work  as  bears 
upon  this  question  will  show  that  there  is  too  much  of 
error  in  the  conclusions  of  Higgins  to  justify  the  claims 
made  for  him.  There  are  some  scattered  allusions  and 
phrases  which  might  be  interpreted  as  glimpses  of  the 
theory.  It  is  stated  that  in  certain  compounds  the  small- 
est particles  of  the  elements  are  contained  in  simple 
numerical  relations  and,  where  there  are  several  com- 
pounds of  the  same  two  elements,  ratios  of  composition 
are  accepted  which  correspond  with  the  law  of  multiple 
proportions.  Thus  Higgins  assumed  that  in  sulphurous 
acid  i  part  by  weight,  in  sulphuric  acid  2  parts  by  weight 
of  oxygen,  to  i  part  by  weight  of  sulphur,  were  to  be 
found.  If  then  the  smallest  particles  of  oxygen  and  sul- 
phur had  the  same  weight  there  were  in  the  two  bodies 
respectively  i  and  2  smallest  particles  of  oxygen  corn- 
combined  with  i  of  sulphur.  So  too  in  nitric  oxide,  he 
maintained  there  were  2  particles  of  oxygen  to  i  of  nitro- 
gen and  hence  2  smallest  particles  of  oxygen  to  i  of 
nitrogen.  In  nitric  acid  there  were  5  particles  of  oxygen 
to  i  of  nitrogen  and  this  he  believed  to  be  the  maximum 
possible  amount  of  oxygen  which  the  i  particle  of  nitro- 
gen could  take  up.  It  cannot  be  maintained  that  Higgins 
always  regarded  the  particles  of  the  elements  as  having 
the  same  weight,  for  in  case  of  water  he  assumed  i  par- 
ticle of  hydrogen  and  i  particle  of  oxygen  to  be  present 
though  the  weights  of  the  two  in  the  compound  were 

1  "A  Comparative  View  of  the  Phlogistic  and  Antiphlogistic  Theories,  with 
Inductions,  &c.,"  1789. 


90  A   STUDY   OF  THE   ATOMS. 

known  to  be  far  from  equal.  Whatever  Higgins  may 
have  thought  of  his  principle  he  nowhere  states  it  as  a 
general  principle  but  gives  a  few  such  instances,  as  those 
mentioned,  scattered  through  his  book.  Hence  it  was 
that  no  chemist  in  the  fifteen  years  that  intervened  be- 
tween the  publication  of  Higgins  and  that  of  Dalton 
seemed  to  have  found  in  the  book  the  outline  even  of  the 
atomic  theory.  After  the  announcement  of  Dalton' s 
theory  Higgins  claimed  to  have  previously  developed  the 
same  views  himself.  While  he  deserved  some  credit  as 
having  caught  glimpses  of  the  atomic  idea,  certainly  he 
can  lay  no  claim  to  having  even  aided  in  the  development 
of  the  atomic  theory.  Unfortunately,  Higgins'  mode  of 
expression  was  so  confused  and  indistinct  that  it  is  not 
always  clear  what  he  meant  nor  how  much  he  knew. 
Part  of  his  work  may  be  interpreted  as  anticipating  the 
discoveries  of  Gay-Lussac  and  the  theory  of  Avogadro, 
and  indeed  has  been  so  interpreted,  were  it  not  for  the 
fact  that  other  portions  of  the  work,  contradict  such  ideas 
and  show  that  something  else  must  have  been  his  mean- 
ing. His  views  as  to  atoms  and  the  constitution  of 
bodies  then  were  confused  or  not  fully  matured,  and  he 
both  failed  to  recognize  their  importance  and  to  attempt 
to  draw  the  attention  of  chemists  to  them. 

It  is  a  matter  of  much  interest  to  trace  the 
Dalton  s  steps  by  which  Dalton  reached  the  conclu- 

sion  that  the  theory  of  atoms  was  the  best 
and  most  satisfactory  explanation  of  the  fundamental 
facts  of  chemistry.  The  laws,  whose  development  we 
have  just  followed,  seem  really  to  have  been  unknown  to 
him  or  to  have  had  little  influence  upon  his  thinking. 
There  are  extant  two  accounts  of  what  led  up  to  the  dis- 
covery. The  one  is  a  conversation  with  Dalton  reported 


ATOMIC   THEORY   OF   CHEMISTRY.  9 1 

by  Thomson  and  the  other  is  one  of  his  own  written  lec- 
tures recently  discovered  by  Roscoe.  A  third  method  of 
getting  at  the  facts  is  by  a  critical  examination  of  his 
published  papers  at  the  period  of  his  discovery.  These 
three  sources  of  information  it  will  be  well  for  us  to  ex- 
amine and  compare  at  some  length. 

Thomson' s  account  is  as  follows1 :  "In 
the  year  1804,  on  the  26th  of  August,  I 
spent  a  day  or  two  at  Manchester  and 
was  much  with  Mr.  Dal  ton.  At  that  time  he  explained 
to  me  his  notions  respecting  the  composition  of  bodies. 
I  wrote  down  at  the  time  the  opinions  which  he  offered, 
and  the  following  account  is  taken  literally  from  my 
journal  of  that  date  :  '  The  ultimate  particles  of  all  bod- 
ies are  atoms  incapable  of  further  division.  These  atoms 
(at  least  viewed  along  with  their  atmosphere  of  heat)  are 
all  spheres  and  are  each  of  them  possessed  of  particular 
weights  which  may  be  denoted  by  numbers.  For  the 
greater  clearness  he  represented  the  atoms  of  the  simple 
bodies  by  symbols.  It  was  this  happy  idea  of  represent- 
ing the  atoms  and  constitution  of  bodies  by  symbols  that 
gave  Mr.  Dalton' s  opinions  so  much  clearness.  I  was 
delighted  with  the  new  light  which  immediately  struck 
my  mind  and  saw  at  a  glance  the  immense  importance  of 
such  a  theory  when  developed.  Mr.  Dalton  informed  me 
that  the  atomic  theory  first  occurred  to  him  during  his 
investigations  of  olefiant  gas  and  carburetted  hydrogen 
gases,  at  that  time  imperfectly  understood,  and  the  com- 
position of  which  was  first  developed  by  Mr.  Dalton  him- 
self. It  was  obvious  from  the  experiments  which  he 
performed  upon  them  that  the  constituents  of  both  were 
carbon  and  hydrogen  and  nothing  else.  He  found,  fur- 

1  Thomson  :  "  History  of  Chemistry,"  II,  289-291. 


92  A   STUDY  OF  THE   ATOMS. 

ther,  that  if  we  reckon  the  carbon  in  each  the  same,  then 
carburetted  hydrogen  gas  contains  exactly  twice  as  much 
hydrogen  as  olefiant  gas  does.  This  determined  him  to 
state  the  ratios  of  these  constituents  in  numbers  and  to 
consider  the  olefiant  gas  as  a  compound  of  i  atom  of 
carbon  and  i  atom  of  hydrogen ;  and  carburetted  hy- 
drogen of  i  atom  of  carbon  and  2  atoms  of  hydrogen. 
The  idea  thus  conceived  was  applied  to  carbonic  oxide, 
water,  ammonia,  etc.,  and  numbers  representing  the 
atomic  weights  of  oxygen,  azote,  etc.,  were  deduced  from 
the  best  analytical  experiments  which  chemistry  then 
possessed.  Let  not  the  reader  suppose  that  this  was  an 
easy  task.  Chemistry  at  that  time  did  not  possess  a  sin- 
gle analysis  which  could  be  considered  as  approaching 
accuracy.  A  vast  number  of  facts  had  been  ascertained 
and  a  fine  foundation  laid  for  future  investigation,  but 
nothing,  as  far  as  weight  and  measure  were  concerned, 
deserving  the  least  confidence  existed.  We  need  not  be 
surprised  then  that  Mr.  Dal  ton's  first  numbers  were  not 
exact.  It  required  infinite  sagacity  and  not  a  little  labor 
to  come  so  near  the  truth  as  he  did.'  " 

It  is  quite  clear  from  this  account  that  Thomson 
thought  the  atomic  theory  resulted  from  the  considera- 
tion of  the  work  with  the  two  hydrocarbons,  but  Dalton's 
statement  is  that  the  idea  came  to  him  at  the  time  when 
he  was  engaged  upon  the  work,  or  rather  was  fully  for- 
mulated then,  and  he  made  use  of  the  example  of  these 
hydrocarbons  to  make  it  plain  to  Thomson.  This  can  be 
shown  to  be  the  case  both  from  his  own  later  account  and 
from  the  consideration  of  his  other  published  papers. 

His  original  lecture  notes  from  which 
Dakon's  Leo  the  second  account  is  taken  are  dated 
ture  Notes. 

February  3,  1810,  and  were  for  a  series 

of  lectures  delivered  before  the  Royal  Institution  of  London. 


ATOMIC   THEORY  OF   CHEMISTRY.  93 

' '  Having  been  long  accustomed  to  make  meteorologi- 
cal observations,  and  to  speculate  upon  the  nature  and 
constitution  of  the  atmosphere,  it  often  struck  me  with 
wonder  how  a  compound  atmosphere,  or  a  mixture  of  two 
or  more  elastic  fluids,  should  constitute  apparently  a 
homogeneous  mass,  or  one  in  all  mechanical  relations 
agreeing  with  a  simple  atmosphere." 

"Newton  had  demonstrated  clearly  in  the  23rd  proposi- 
tion of  Book  2  of  the  '  Principia,'  that  an  elastic  fluid  is 
constituted  of  small  particles  or  atoms  of  matter  which 
repel  each  other  by  a  force  increasing  in  proportion  as 
their  distance  diminishes.  But  modern  discoveries  have 
ascertained  that  the  atmosphere  contains  three  or  more 
elastic  fluids  of  different  gravities  ;  it  did  not  appear  to 
me  how  this  proposition  of  Newton  would  apply  to  a 
case  of  which  he,  of  course,  could  have  no  idea. 

"The  same  difficulty  occurred  to  Dr.  Priestley,  who 
discovered  this  compound  nature  of  the  atmosphere.  He 
could  not  conceive  why  the  oxygen  gas  being  specifically 
heavier,  should  not  form  a  distinct  stratum  of  air  at  the 
bottom  of  the  atmosphere  and  the  azotic  gas  one  at  the 
top  of  the  atmosphere.  Some  chemists  upon  the  conti- 
nent, I  believe  the  French,  found  a  solution  of  this  diffi- 
culty (as  they  apprehended).  It  was  chemical  affinity. 
One  species  of  gas  was  held  in  solution  by  the  other ;  and 
this  compound  in  its  turn  dissolved  water  ;  hence  evapora- 
tion, rain,  etc.  This  opinion  of  air  dissolving  water  had 
long  before  been  the  prevailing  one,  and  naturally  paved 
the  way  for  the  reception  of  that  which  followed,  of  one 
kind  of  air  dissolving  another.  It  was  objected  that  there 
were  no  decisive  marks  of  chemical  union  when  one  kind 
of  air  was  mixed  with  another — the  answer  was  that  the 
affinity  was  of  a  very  slight  kind,  npt  of  that  energetic 
cast  that  is  observable  in  most  other  cases." 


94  A  STUDY  OF  THE  ATOMS. 

Dalton  then  described  at  some  length  his  efforts  at 
adapting  the  ' '  chemical  theory  of  the  atmosphere  to  the 
Newtonian  doctrine  of  repulsive  atoms  or  particles." 
He  continues  :  "  In  1801  I  hit  upon  an  hypothesis  which 
completely  obviated  the  difficulties.  According  to  this 
we  were  to  suppose  that  the  atoms  of  one  kind  did  not  re- 
pel the  atoms  of  another  kind  but  only  those  of  their  own 
kind.  This  hypothesis  most  effectually  provided  for  the 
diffusion  of  any  one  gas  through  another,  whatever  might 
be  their  specific  gravities  and  perfectly  reconciled  any 
mixture  of  gases  to  the  Newtonian  theorem.  Every 
atom  of  both  or  all  of  the  gases  in  the  mixture  was  the 
center  of  repulsion  to  the  proximate  particles  of  its  own 
kind,  disregarding  those  of  the  other  kind.  All  the  gases 
united  in  their  efforts  in  counteracting  the  pressure  of  the 
atmosphere  or  any  other  pressure  that  might  be  opposed 
to  them.  This  hypothesis,  however  beautiful  might  be 
its  application,  had  some  improbable  features. 

"  We  were  to  suppose  as  many  distinct  kinds  of  repul- 
sive powers  as  of  gases  ;  and  moreover,  to  suppose  that 
heat  was  not  the  repulsive  power  in  any  one  case  ;  posi- 
tions certainly  not  very  probable.  Besides  I  found  from 
a  train  of  experiments,  which  have  been  published  in  the 
1  Manchester  Memoirs/  that  the  diffusion  of  gases 
through  each  other  was  a  slow  process,  and  appeared  to 
be  a  work  of  considerable  effort.  Under  reconsidering 
the  subject,  it  occurred  to  me  that  I  had  never  contem- 
plated the  effect  of  difference  of  size  in  the  particles  of 
elastic  fluids.  By  size  I  mean  the  hard  particles  at  the 
center  and  the  atmosphere  of  heat  taken  together.  If, 
for  instance,  there  be  not  exactly  the  same  number  of 
atoms  of  oxygen  in  a  given  volume  of  air  as  of  azote  in 
the  same  volume,  then  the  sizes  of  the  particles  of  oxygen 


ATOMIC  THEORY  OF  CHEMISTRY.  95 

must  be  different  from  those  of  azote.  And  if  the  sizes 
be  different,  then  on  the  supposition  that  the  repulsive 
power  is  heat,  no  equilibrium  can  be  established  by  parti- 
cles of  unequal  sizes  pressing  against  each  other. 

"This  idea  occurred  to  me  in  1805.  I  soon  found  that 
the  sizes  of  the  particles  of  elastic  fluids  must  be  different; 
for  a  measure  of  azotic  gas  and  one  of  oxygen,  if  chemi- 
cally united,  would  make  nearly  two  measures  of  nitrous 
gas,  and  those  two  could  not  have  more  atoms  of  nitrous 
gas  than  the  one  measure  had  of  azote  or  oxygen.  Hence 
the  suggestion  that  all  gases  of  different  kinds  have  a  dif- 
ference in  the  size  of  their  atoms,  and  thus  we  arrive  at 
the  reason  for  that  diffusion  of  every  gas  through  every 
other  gas,  without  calling  in  any  other  repulsive  power 
than  the  well-known  one  of  heat.  This  then  is  the  pres- 
ent view  which  I  have  of  the  constitution  of  a  mixture  of 
elastic  fluids.  The  different  sizes  of  the  particles  of 
elastic  fluids  under  like  circumstance  of  temperature  and 
pressure  being  once  established,  it  became  an  object  to 
determine  the  relative  sizes  and  weights,  together  with 
the  relative  number  of  atoms  in  a  given  volume.  This 
led  the  way  to  the  combination  of  gases  and  to  the  num- 
ber of  atoms  entering  into  such  combinations,  the  par- 
ticulars of  which  will  be  detailed  more  at  length  in  the 
sequel.  Other  bodies  besides  elastic  fluids,  namely, 
liquids  and  solids,  were  subject  to  investigation  in  con- 
sequence of  their  combining  with  elastic  fluids.  Thus  a 
train  of  investigation  was  laid  for  determining  the  num- 
ber and  weight  of  all  chemical  elementary  principles  which 
enter  into  any  sort  of  combination,  one  with  another."1 

As  Roscoe  and  Harden  remark,  it  may  be  well  to  re- 
member that  according  to  Dalton's  view,  which  is  a  modi- 

1  Roscoe  and  Harden  :  "  New  View  of  Dalton's  Atomic  Theory,"  p.  13. 


96  A   STUDY   OF  THE   ATOMS. 

fication  of  that  of  Newton  and  Lavoisier,  each  atom  or 
particle  of  a  gas  consisted  of  an  exceedingly  small  central 
nucleus  of  solid  matter  surrounded  by  an  enormously 
more  bulky  elastic  atmosphere  of  heat,  of  great  density 
next  the  atom,  but  gradually  growing  rarer  according  to 
some  power  of  the  distance.  To  this  atmosphere  of  heat 
was  ascribed  the  power  of  repulsion  by  means  of  which 
the  elastic  state  of  the  gas  was  maintained.  By  increasing 
the  amount  of  heat  round  each  atom  the  density  of  the 
gas  would  therefore  be  diminished.1 

The  same  authors  observe  that  the  date  1805  given 
above  by  Dalton  must  be  a  clerical  error  for  1803  since  he 
had  communicated  an  account  of  the  atomic  theory  to 
Thomson  in  1804  and  as  can  be  seen  from  his  note-books 
had  worked  out  a  table  of  the  diameters  of  the  atoms  in 
September,  1803. 

To  complete  the  view  of  the  incep- 

£fuuctoons  from     tion  of  the  modern  atomic  theory {t 

Other  Papers.  * 

is  necessary  now   to   consider   the 

early  papers  published  by  Dalton  which  bear  upon  this 
subject.  Dalton' s  training  was  more  especially  that  of  a 
mathematician  and  physicist,  and  he  was  particularly  in- 
terested in  meteorological  observations  and  the  phenom- 
ena of  gases.  In  1793  he  published2  his  first  researches 
having  for  their  object  the  elucidations  of  certain  meteoro- 
logical points,  especially  the  moisture  in  the  atmosphere 
and  the  conditions  under  which  this  water  vapor  existed 
there.  This  question  seemed  to  have  been  one  of  pecu- 
liar fascination  and  interest  for  him.  Eight  years  later 
(i8oi)8  he  published  a  paper  on  the  "Constitution  of 
Mixed  Gases."  In  this  he  asserted  that  the  total  pres- 

1  Roscoe  and  Harden,  p.  19. 

2  4<  Meteorological  Observations  and  Essays,"  Manchester,  1793. 

3  Memoirs,  Manchester  Ut.  and  Phil.  Soc.,  V,  535. 


ATOMIC  THEORY   OF   CHEMISTRY.  97 

sure  of  a  mixture  of  two  gases  on  the  walls  of  the  con- 
taining vessel  is  equal  to  the  sum  of  the  pressures  of  each 
gas  ;  if  one  gas  is  removed,  the  pressure  now  exerted  by 
the  remaining  gas  is  exactly  the  same  as  was  exerted  by 
that  gas  in  the  original  mixture.  The  variations  in  the 
pressure  of  various  gases  caused  by  increasing  and  de- 
creasing temperature  were  considered  and  the  relations 
which  exist  between  the  volumes  of  gases  and  the  tem- 
perature at  which  these  volumes  were  measured.  As  a 
mathematician  the  idea  of  Bernoulli  was  probably 
known  to  him  that  the  pressure  exerted  by  a  gas  on  the 
walls  of  a  vessel  enclosing  it  was  due  to  the  constant 
bombardment  of  the  walls  by  the  atoms  of  which  the  gas 
consisted. 

Dalton  says  of  this  paper,  in  a  second  memoir1  pub- 
lished in  1802  :  "  My  principal  object  in  that  essay  was 
to  point  out  the  manner  in  which  elastic  fluids  exist  to- 
gether, and  to  insist  upon  what  I  think  is  a  very  impor- 
tant and  fundamental  position  in  the  doctrine  of  such  fluids, 
namely,  that  the  elastic  or  repulsive  power  of  each  parti- 
cle is  confined  to  those  of  its  own  kind  and  consequently 
the  force  of  such  fluid,  retained  in  a  given  vessel,  or  grav- 
itating, is  the  same  in  a  separate  as  in  a  mixed  state,  de- 
pending upon  its  proper  density  and  temperature. ' ' 

Dalton  read  on  November  12,  1802,  a  paper,2  entitled 
"An  Experimental  Enquiry  into  the  Properties  of  the 
Several  Gases  or  Elastic  Fluids,  Constituting  the  Atmos- 
phere. ' '  He  set  forth  his  aim  in  this  research  as  follows  : 

i .  To  determine  the  weight  of  each  simple  atmosphere 
abstractedly,  or,  in  other  words,  what  part  of  the  weight 
of  the  whole  compound  atmosphere  is  due  to  azote  ;  what 
to  oxygen,  etc. 

1  Memoirs  Manchester,  I,it.  and  Phil.  Society,  1802. 
3  Manchester  Memoirs,  I,  pp.  248,  249. 


98  A  STUDY  OF  THE   ATOMS. 

2.  To  determine  the  relative  weights  of  the  different 
gases  in  a  given  volume  of  atmospheric  air,  such  as  it  is 
at  the  earth's  surface. 

3.  To  investigate  the  properties  of  the  gases  to  each 
other,  such  as  they  ought  to  be  found  at  different  eleva- 
tions above  the  earth's  surface. 

In  this  memoir  he  clearly  states  his  belief  that  the  at- 
mosphere was  not  a  chemical  compound.  In  connection 
with  his  careful  working  out  of  the  proportions  by  weight 
of  the  constituents  of  the  atmosphere,  it  should  be  remem- 
bered that  the  work  of  Gay-Lussac  and  Humboldt  upon 
the  analysis  of  the  air  was  not  presented  before  the  French 
Academy  until  three  years  later,  in  1805.  In  another 
paper  at  this  time  Dal  ton  showed  that  all  gases  expanded 
alike  from  heat,  and  that  this  expansion  was  very  nearly 
i/48oth  of  the  volume  for  each  additional  i°  F.  In  this 
he  again  anticipated  Gay-Lussac  in  his  classic  work  upon 
the  same  subject.  While  Dalton's  main  results  in  these 
investigations  have  apparently  little  direct  bearing  upon 
the  subject  under  discussion,  they  are  briefly  mentioned 
here  to  show  the  trend  of  his  work  and  thoughts.  But 
there  is  one  portion  of  his  '  *  Enquiry  into  the  Properties 
of  Elastic  Fluids  "  which  has  a  very  direct  bearing  upon 
the  subject,  giving  the  first  glimpse  of  the  law  of  multi- 
ple proportions.  In  determining  the  amount  of  oxygen 
in  the  atmosphere,  the  following  experiment  was  per- 
formed : 

"  If  100  measures  of  common  air  be  put  to  36  of  pure 
nitrous  gas  in  a  tube  0.3  inch  wide  and  5  inches  long, 
after  a  few  minutes  the  whole  will  be  reduced  to  79  or  80 
measures  and  exhibit  no  signs  of  either  oxygenous  or  ni- 
trous gas.  If  loo  measures  of  common  air  be  admitted  to 
72  of  nitrous  gas  in  a  wide  vessel  over  water,  such  as  to 


ATOMIC  THEORY   OF   CHEMISTRY.  99 

form  a  thin  stratum  of  air,  and  an  immediate  momentary 
agitation  be  used,  there  will,  as  before,  be  found  79  or  80 
measures  of  pure  azotic  gas  for  a  residuum. 

"If  in  this  last  experiment  less  than  72  measures  of  ni- 
trous gas  be  used,  there  will  be  a  residuum  containing  ox- 
ygenous gas;  if  more,  then  some  residuary,  nitrous  gas,  will 
be  found.  These  facts  clearly  point  out  the  theory  of  the 
process  ;  the  elements  of  oxygen  may  combine  with  a 
certain  portion  of  nitrous  gas,  or  with  twice  that  portion, 
but  with  no  intermediate  quantity.  In  the  former  case, 
nitric  acid  is  the  result,  in  the  latter,  nitrous  acid." 

With  regard  to  this  experiment  Roscoe  says  :l  "In  the 
memorable  case  in  which  Dalton  announces  the  first 
instance  of  combination  in  multiple  proportions,  the  whole 
conclusion  is  based  upon  an  erroneous  experimental  basis. 
If  we  repeat  the  experiment,  as  described  by  Dalton,  we 
do  not  obtain  the  results  he  arrived  at.  We  see  that 
Dal  ton's  conclusions  were  correct,  although  in  this  case 
it  appears  to  have  been  a  mere  chance  that  his  experi- 
mental results  rendered  such  a  conclusion  possible. ' ' 

I  have  seen  no  suggestions  as  to  what  Dalton  meant  by 
the  "elements  of  oxygen"  in  the  passage  cited  above. 
The  word  'elements'  seems  meaningless  unless  he  was  here 
thinking  of  the  component  particles  of  this  gas  which  he 
well  recognized  was  not  compound.  At  the  same  time 
he  knew  that  nitrous  oxide  was  compound,  and  so  this 
experiment  did  not  have  the  simplicity  of  his  next  ex- 
ample of  multiple  proportions  in  which  he  was  dealing 
with  carbon  and  hydrogen  alone. 

A  paper2  on  the  '  'Absorption 

«*.  G- by  wate; rdnrer 

Liquids    was  read  by  Dalton, 
before  the   Manchester  Society  on   October   21,    1803. 

1  Roscoe  :  Chtm.  News,  30,  266-267. 

2  Manchester  Memoirs,  1805. 


100  A   STUDY   OF  THE   ATOMS. 

There  are  fifteen  propositions  made  in  this  article  ;  some 
statements  of  well-known  facts,  others  the  result  of  ex- 
periments performed  by  Dal  ton  and  Henry.  Upon  these 
was  built  a  mechanical  theory  of  the  absorption  of  gases. 
In  this  discussion  there  is  frequent  reference  to  '  'particles 
of  gas. ' '  Thus,  '  'A  particle  of  gas  pressing  on  the  sur- 
face of  water  is  analogous  to  a  single  shot  pressing  upon 
the  summit  of  a  square  pile  of  them  ; ''  or  again,  "each 
particle  of  gas  must  divide  its  force  equally  amongst  a 
number  of  particles  of  water. ' '  The  article  closes  with 
the  following  noteworthy  sentences: 

"The  greatest  difficulty  attending  the  mechanical  hy- 
pothesis arises  from  different  gases  observing  different 
laws.  Why  does  water  not  admit  its  bulk  of  every  kind 
of  gas  alike  ?  This  question  I  have  duly  considered  and 
though  I  am  not  yet  able  to  satisfy  myself  completely  I 
am  nearly  persuaded  that  the  circumstance  depends  upon 
the  weight  and  number  of  the  ultimate  particles  of  the 
several  gases  ;  those  whose  particles  are  lightest  and 
single  being  least  absorbable,  and  the  others  more,  accord- 
ing as  they  increase  in  weight  and  complexity  (he  added 
in  a  foot-note:  'Subsequent  experiment  renders  this  con- 
jecture less  probable').  An  inquiry  into  the  relative 
weights  of  the  ultimate  particles  of  bodies  is  a  subject,  as 
far  as  I  know,  entirely  new.  I  have  lately  been  prose- 
cuting this  inquiry  with  remarkable  success.  The  prin- 
ciple cannot  be  entered  upon  in  this  paper,  but  I  shall 
just  subjoin  the  results,  as  far  as  they  appear  to  be  ascer- 
tained by  my  experiments. ' ' 

TABI^E  OF  THE  RELATIVE  WEIGHTS  OF  THE  ULTIMATE  PARTICLES 
OF  GASEOUS  AND  OTHER  BODIES. 

.   Hydrogen i.o 

Azot 4.2 

Carbone 4.3 


ATOMIC   THEORY   OF   CHEMISTRY.  IOI 

Ammonia 5.2 

Oxygen 5.5 

Water 6.5 

Phosphorus    7.2 

Phosphuretted  hydrogen 8.2 

Nitrous  gas 9.3 

Ether 9.6 

Gaseous  oxide  of  carbone 9.8 

Nitrous  oxide 13.7 

Sulphur 14.4 

Nitric  acid 15.2 

Sulphuretted  hydrogen 15.4 

Carbonic  acid 15.3 

Alcohol 15.1 

Sulphureous  acid 19.9 

Sulphuric  acid 25.4 

Carburetted  hydrogen  from  stagnant  water 6.3 

Olefiant  gas 5.3 

While  this  and  the  previous  paper 

A'tomic  Weights.         tear  the  date  '8°2'  the  volume  of 
the  memoirs    of  the    Manchester 

Literary  and  Philosophical  Society  containing  them  was 
not  published  until  1805,  and  there  is  good  reason  for 
believing  that  during  these  three  years,  in  which  they  lay 
unpublished,  Dalton  added  to  them  such  new  facts  and 
conclusions  as  occurred  to  him  and  seemed  necessary  to 
bring  them  up  to  date.  Roscoe  has  shown  conclusively 
from  the  testimony  of  Dalton' s  laboratory  note-book,  that 
he  was  still  experimenting  in  1803  as  if  he  were  in  igno- 
rance of  the  remarkable  experiment  described  on  page  92 
which  gave  the  first  recorded  case  of  multiple  proportions. 
This  experiment  itself  is  given  later  in  the  note-books 
but  unfortunately  without  date. 

And  so  with  regard  to  the  table  of  weights  given  above, 
it  may  be  fairly  concluded  that  this  was  not  the  original 
table  but  a  later  corrected  one,  for  Roscoe  and  Harden1 

1  Roscoe  and  Harden,  p.  28. 


102  A  STUDY   OF  THE   ATOMS. 

have  found  in  the  same  note-books  under  date  September 

6,  1803,  or  some  six  weeks  before  the  reading  of  the  paper 

before  the  Society,  the  following  table  which  seems  to  be 

the  first  attempt  at  a  table  of  the  atomic  weights. 

DAI/TON'S  FIRST  ESSAY  AT  A  TABLE  OF  ATOMIC  WEIGHTS, 

SEPTEMBER  6,  1803. 

Ult.  at.  Hydrogen i.o 

Oxygen 5.66 

Azot 4.0 

Carbon  (charcoal)  4.5 

Water 6.66 

Ammonia*... 5.0 

Nitrous  gas 9.66 

Nitrous  oxide 13.66 

Nitric  acid 15.32 

Sulphur 17.00 

Sulphureous  acid 22.66 

Sulphuric  acid 28.32 

Carbonic  acid 15.8 

Oxide  of  carbone 10.2 

There  is  no  evidence  from  the  note-books  of  the  con- 
struction of  the  other  table  at  or  near  the  time  given  at 
the  heading  of  the  paper.  Dalton  was  secretary  of  the 
Society  and  of  course  had  abundant  opportunity  to  insert 
such  changes  as  he  saw  fit.  There  could  have  been  no 
question  in  his  mind  as  to  priority  or  historical  claims, 
his  aim  being  simply  scientific  accuracy. 

The  table  is  worthy  of  careful  study.  The  mention  of 
' '  ultimate  atoms  ' '  is  found  here  and  elsewhere  in  these 
note-books  of  the  same  date.  Dalton' s  theory  was  at 
first  evidently  corpuscular  like  that  of  Newton.  His 
atom  was  not  an  indivisible  unit  but  a  particle  or  little 
mass.  It  grew  into  an  atomic  theory  with  greater  knowl- 
edge. Again,  we  have  here  three  distinct  cases  of  multi- 
ple proportions  and  yet  there  is  no  reference  to  the  case 
of  olefiant  gas  and  carburet  ted  hydrogen  which,  from  the 


ATOMIC  THEORY   OF   CHEMISTRY.  103 

conversation  with  Thomson,  was  the  first  or  at  least  the 
deciding  case  which  led  him  both  to  the  law  of  multiple 
proportions  and  the  atomic  theory. 

It  is  clear  from  all  that  has  been  said  that 
Dalton's  takinS  UP  of  the  atomic 


was  no  sudden  inspiration  springing  from 
some  newly  acquired  fact,  but  a  matter  of  slow  growth, 
coming  first  from  his  meteorological  and  mathematical 
studies  and  in  particular  from  his  thinking  over  the  prob- 
lem connected  with  the  gases  of  the  atmosphere  ;  and  that 
he  thought  long  and  deeply  over  these  problems,  declining 
to  accept  the  usual  explanations  and  gradually  substitu- 
ting the  corpuscular  theory.  His  chemical  experiments 
enabled  him  to  very  wonderfully  substantiate  this  theory, 
but  the  accepted  theory  of  elements  made  it  necessary  to 
convert  the  corpuscular  into  an  atomic  theory.  It  is 
doubtful  whether  Dalton  thought  out  or  cared  about  the 
difference  between  the  two  ideas,  for,  after  all,  his  theory 
was  at  the  beginning  very  simple  and  crude.  It  seems  to 
be  impossible  to  fix  upon  any  exact  date  for  the  inception 
of  the  atomic  theory,  but  the  date  most  worthy  of  being 
so  accepted  would  be  the  one  given  in  his  note-book  along 
with  the  first  table  of  atomic  weights,  namely,  September 
6,  1803.  Henry1  sums  up  the  evidence  known  to  him  as 
follows  :  "  My  own  belief  is  that  during  the  three  years 
(1802-1804)  in  which  the  main  foundations  of  the  atomic 
theory  were  laid,  Dalton  had  patiently  and  maturely  re- 
flected on  all  the  phenomena  of  chemical  combination 
known  to  him  from  his  own  researches  or  those  of  others, 
and  had  grasped  in  his  comprehensive  survey,  as  signifi- 
cant to  him  of  a  deeper  meaning  than  to  his  predecessors, 
their  empirical  laws  of  constant  and  reciprocal  propor- 

i  Henry  :  ••  Ufe  of  Dalton,"  p.  85. 


104  A   STUDY   OP   THE   ATOMS. 

tions,  and  his  own  researches  in  the  chemistry  of  aeriform 
bodies. ' '  Henry  adds  that  after  the  lapse  of  twenty  years 
Dalton  himself  may  have  failed  in  recalling  the  antece- 
dents of  his  great  discovery  in  the  exact  order  of  sequence. 
If  individual  judgment  has  any  more  value  in  such  a  mat- 
ter than  a  mere  guess,  it  might  be  suggested  that  the  account 
in  his  lecture  of  1810  is  the  result  of  the  mature  and  de- 
liberate sifting  of  the  earlier  thoughts  and  beliefs,  whereas 
in  the  conversation  of  1804,  with  a  desire  for  bringing 
about  conviction  in  his  hearer,  he  gave  as  the  foundation 
of  his  theory  the  facts  which  had  been  most  recently  ac- 
quired and  so  most  impressed  him  and  offered  the  best 
means  of  making  his  meaning  clear. 

Thomson's  evidence  is  direct  and  conclusive  as  to  Dai- 
ton's  independence  of  the  previous  work  of  Richter.  He 
says  :  *  *  I  do  not  know  when  he  adopted  these  notions 
(*.  £.,  the  atomic  theory),  but  when  I  visited  him  in  1804 
at  Manchester  he  had  adopted  them,  and  at  that  time 
both  Mr.  Dalton  himself  and  myself  were  ignorant  of 
what  had  been  done  by  Richter  on  the  same  subject."1 

This  discussion  may  well  be  con- 
Conclusions  of  d  ded     .  h  the  remarks  of  Ros. 

Roscoe  and  Harden.  „  ..mi 

coe  and  Harden:       There  seems 

to  be  no  doubt  that  the  idea  of  atomic  structure  arose  in 
Dalton' s  mind  as  a  purely  physical  conception,  forced 
upon  him  by  his  study  of  the  physical  properties  of  the 
atmosphere  and  other  gases.  Confronted  in  the  course  of 
this  study  with  the  problem  of  ascertaining  the  relative 
diameters  of  the  particles,  of  which  he  was  firmly  con- 
vinced all  gases  were  made  up,  he  had  recourse  to  the 
results  of  chemical  analysis.  Assisted  by  the  assumption 
that  combination  always  takes  place  in  the  simplest  pos- 

1  Proc.  Glasgow  Phil.  Soc.,  1845-46,  p.  86. 

2  Roscoe  and  Harden  :  "  New  View  of  Dalton's  Atomic  Theory,"  p.  50. 


ATOMIC  THEORY  OF   CHEMISTRY.  105 

sible  way,  he  thus  arrived  at  the  idea  that  chemical  com- 
bination takes  place  between  particles  of  different  weights, 
and  this  it  was  which  differentiated  his  theory  from  the 
historic  speculations  of  the  Greeks.  The  extension  of  this 
idea  to  substances  in  general  led  him  to  the  law  of  com- 
bination in  multiple  proportions,  and  the  comparison  with 
experiment  brilliantly  confirmed  the  truth  of  his  deduc- 
tion. Once  discovered,  the  principle  of  atomic  union  was 
found  to  be  of  universal  application.  Nothing  essential 
has  since  been  added  to  our  knowledge  of  the  laws  of 
chemical  combination  by  weight.  To  Dalton  must  be 
ascribed  the  rare  merit  of  having  by  the  application  of  a 
single  felicitous  idea  to  a  whole  class  of  the  facts  of  chem- 
istry, so  completely  comprehended  the  prevailing  rela- 
tions that  his  generalizations  have  sustained  without  al- 
teration the  labors  and  changes  of  almost  an  entire  century. 

The  details  of  Dal  ton's  atomic  the- 

Daltln'sTheory.        "J  were  very  fewjmd  f£mpk;    ^ 
did  not  concern  himself  with  the 

vexed  questions  concerning  these  atoms  with  which  the 
centuries  struggled.  The  following  statements  may  be 
gathered  from  his  ' *  New  System  of  Chemical  Philoso- 
phy." 

1.  All  bodies  of  sensible  magnitude  are  constituted  of  a 
vast  number  of  extremely  small  particles  or  atoms  of 
matter  bound  together  by  a  force  of  attraction  which,  as 
it  endeavors  to  prevent  their  separation,  is  called  attrac- 
tion of  cohesion  ;  but  as  it  collects  them  from  a  dispersed 
state  is  called  attraction  of  aggregation  or  more  simply 
affinity.1 

2.  The  ultimate  particles  of  all  homogeneous  bodies 
are  perfectly  alike  in  weight,  figure,  etc.     In  other  words 

1  Dalton 's  "  System  of  Chemical  Philosophy,"  p.  143. 


106  A   STUDY  OF  THE  ATOMS. 

every  particle  of  water  is  like  every  other  particle  of 
water ;  every  particle  of  hydrogen  is  like  every  other 
particle  of  hydrogen  ;  etc.1 

3.  No  new  creation  or  destruction  of  matter  is  within 
the  reach  of  chemical  agency.     All  the  elements  we  can 
produce  consist  in  separating  particles  that  are  in  a  state 
of  cohesion  or  combination  and  joining  those  that  were 
previously  at  a  distance.2 

4.  The  ultimate  particles  of  all  simple  bodies  are  atoms 
incapable  of  further  division.     These  atoms   (at  least 
viewed  along  with  their  atmospheres   of  heat)  are  all 
spheres  and  are  possessed  of  particular  weights  which  may 
be  denoted  by  number.3 

5.  If  there  are  two  bodies  which  are  disposed  to  com- 
bine, then  their  combination  takes  place  by  atoms.4 

6.  In  an  elastic  gas  each  particle  occupies  the  center  of 
a  comparatively  large  sphere  and  supports  its  dignity  by 
keeping  all  the  rest,  which  by  their  gravity  or  otherwise 
are  disposed  to  encroach  upon  it,  at  a  respectful  distance.6 

It  will  be  observed  that  such  questions  as  the  existence 
of  vacua,  filling  of  space,  inherent  motion  of  the  particles, 
etc.,  are  left  without  mention.  And  it  was  well  for  the 
atomic  theory  to  begin  life  again  clothed  with  as  few  of 
these  debatable  notions  as  possible.  The  simplicity  of 
Dalton's  statement  is  therefore  praiseworthy.  It  is 
scarcely  necessary  to  call  attention  to  its  crudities. 

Dalton's  papers  read  before  the  Man- 
Chester  Society  seem  to  have  attracted 
but  little  attention.  They  really  con- 

1  Dalton's  "  New  System,"  p.  141. 

2  Dalton's  "New  System,"  p.  212. 

3  Thomson's  "  History  of  Chem.,"  p.  289. 

4  Dalton's  "New  System,"  p.  216. 
6  Dalton's  "New  System,"  p.  211. 


ATOMIC  THEORY  OF   CHEMISTRY.  107 

tained  no  clear  definite  announcement  of  the  atomic  the- 
ory and  in  the  main  were  filled  with  other  matters.  It 
is  only  in  the  light  of  later  events  that  we  can  pick  out 
here  and  there  from  the  earlier  papers  sentences  presaging 
the  coming  theory.  And  these  papers  did  not  reach  the 
larger  circle  of  scientific  readers  outside  as  they  were  not 
published  for  some  years  after  they  were  read  before  the 
society.  It  was  mainly  through  Thomson  that  Dalton's 
conclusions  were  made  known  to  chemists.  He  gave  a 
sketch  of  the  theory  in  his  ' 'System  of  Chemistry,"  pub- 
lished in  1807.  In  the  same  year  he  published  in  the 
Philosophical  Transactions  a  paper  giving  an  example  of 
multiple  proportions.  This  paper  was  on  oxalic  acid  and 
in  it  Thomson  showed  that  oxalic  acid  united  in  two  pro- 
portions with  strontium  and  that,  supposing  the  strontium 
in  both  salts  to  be  represented  by  the  same  amount,  then 
the  oxalic  acid  in  one  is  twice  as  much  as  in  the  other. 

A  few  months  later,  Wollaston  read  before  the  Royal 
Society  of  L,ondon  a  paper  upon  peracid  and  subacid  salts 
in  which  he  showed  how  the  law  of  multiples  was  further 
exemplified  in  the  alkaline  carbonates  and  bicarbonates, 
potassium  sulphate  and  bisulphate,  and  potassium  oxa- 
lates.  To  this  article  Wollaston  appended  some  note- 
worthy observations  upon  the  arrangement  of  atoms  in 
space  which  will  be  referred  to  later.  These  publications 
gradually  drew  the  attention  of  chemists  to  Dalton's 
views.  Some  of  the  most  eminent  chemists,  however, 
were  very  hostile  to  the  theory.  Sir  Humphry  Davy 
was  particularly  opposed  to  it  and  even  descended  to  car- 
icaturing and  ridiculing  it.  But  Wollaston  and  Thomson 
and  Gilbert  were  won  over  and  the  latter  convinced  Davy 
so  that  he  too  became  a  strenuous  supporter  of  the 
theory. 


IO8  A   STUDY   OF  THE   ATOMS. 

The  chemist  who  did  most  for 
Extension  of  the  The-         .,  £   ,,      , 

ory  by  Berzelius.  the  extenslon  <>f   the  law  of 

multiple  proportions,  the  de- 
termination of  atomic  weights  and  the  development  of 
the  theory,  was  Berzelius.  From  1810  on,  its  acceptance 
became  general  among  chemists.  It  was  in  his  "New 
System  of  Chemical  Philosophy,"  in  1808,  that  Dalton 
first  gathered  together  his  views  as  to  the  atoms.  They 
were  placed  under  the  heading  "  Chemical  Synthesis" 
and  formed  the  third  portion  of  the  book,  though  occupy- 
ing altogether  only  a  few  pages.  In  this  he  does  not  give 
facts  upon  which  he  based  the  theory  but  simply  ex- 
presses his  conclusions.  He  introduced  his  symbols 
which  were  somewhat  cumbrous  and  were  afterwards  re- 
placed by  the  symbols  of  Berzelius  which  are  practically 
those  at  present  in  use.  Dalton's  introduction  of  sym- 
bols was  a  most  important  advance  and  rendered  his  the- 
ory much  clearer.  He  appended  a  table  giving  the  sym- 
bols and  atomic  weights  of  37  bodies,  20  of  which  were 
then  considered  simple.  A  few  of  these  are  given  here  to 
show  the  general  character  of  his  numbers. 

Hydrogen i  Phosphorus 9 

Azote 5  Sulphur 13 

Carbon 5  iron 38 

Oxygen 7  Copper 56,  etc. 

He  chose  hydrogen  as  the  standard  because  it  was  the 
lightest  of  all  bodies.  He  thought  all  the  atomic  weights 
of  other  bodies  to  be  most  probably  multiples  of  hydrogen 
and  so  expressed  them  by  whole  numbers. 

In  1810  the  second  volume  of  Dalton's  "New  System 
of  Chemical  Philosophy"  appeared.  It  was  mainly  con- 
cerned with  ingenious  efforts  at  determining  the  atomic 
weights  and  he  gave  a  new  table  of  these  weights,  fuller 
than  the  preceding  one  but  still  very  faulty. 


ATOMIC  THEORY   OF   CHEMISTRY.  109 

The  third  volume  did  not  appear  until  1827.  It  con- 
tained a  new  table  of  atomic  weights  and  in  it  he  still 
adhered  to  his  ratio  of  i :  7  for  hydrogen  and  oxygen,  re- 
fusing to  accept  the  more  accurate  results  of  other  chem- 
ists. 

Dal  ton   had   recognized   that   the 
Dalton's  Rules  for       fi  k  f    ;         h      .       .      h 

Determining  the 
Atomic  Weights.          hSht  of  the  new  theory  was  the 

determination     of     the      relative 

weights  of  the  atoms.  This  was  to  be  accomplished 
by  correct  analyses  of  well -characterized  compounds 
which  gave  the  most  direct  ratios.  He  made  use  not 
only  of  his  own  analyses  but  of  the  best  work  of 
others  known  to  him.  One  of  the  most  serious  problems 
connected  with  this  work  was  that  of  determining  the 
number  of  atoms  in  the  various  compounds.  For  this 
purpose  Dalton  laid  down  a  number  of  arbitrary  rules, 
proceeding  upon  the  assumption  that  nature  always 
worked  in  the  simplest,  most  direct  manner,  an  assump- 
tion which  is  far  from  justifiable  in  the  sense  accepted  by 
Dalton.  His  rules  were  as  follows  : 

1 .  When  only  one  combination  of  two  bodies  can  be 
obtained,  it  must  be  presumed  to  be  a  binary  one,  unless 
some  cause  appear  to  the  contrary. 

2.  When  two  combinations  are  observed,  they  must  be 
presumed  to  be  a  binary  and  a  ternary. 

3.  When  three  combinations  are  obtained,  we  may  ex- 
pect one  to  be  binary  and  the  other  two  ternary. 

4.  When  four  combinations  are  observed,   we  should 
expect  one  binary,  two  ternary  and  one  quaternary. 

A  binary  compound  meant  one  of  2  atoms,  ternary  of 
3  atoms,  quaternary  of  4  atoms,  etc. 


IIO  A  STUDY   OF  THE   ATOMS. 

He  also  adopted  as  a  principle  the  theory  that  the 
atomic  weights  were  all  multiplies  of  hydrogen  and  there- 
fore whole  numbers.  Consequently  in  his  later  tables, 
all  fractions  were  rounded  off  to  the  nearest  integers.  His 
numbers  were  very  faulty  and  after  1810  found  little  ac- 
ceptance. 

Far  better  work  was  done  by  Berzelius,  though  he  also 
found  it  necessary  to  adopt  arbitrary  rules  for  telling  the 
number  of  atoms  in  a  given  compound.  His  rules  may 
be  briefly  stated  thus  :  Summing  up  all  of  his  experiments 
and  investigations  he  believed  that  the  following  rules 
could  be  deduced : 

If  an  element  forms  several  oxides,  and  the  quantities 
of  oxygen  contained  in  them,  as  compared  with  a  fixed 
quantity  of  the  element,  are  to  each  other  as  1:2,  then  it 
is  to  be  concluded  that  the  first  compound  consists  of  i 
atom  of  the  element  and  i  atom  of  oxygen  ;  the  second  of 
i  atom  of  the  element  and  2  atoms  of  oxygen  (or  2  atoms 
of  the  element  and  4  atoms  of  oxygen).  If  the  ratio  is 
2 : 3,  then  the  first  compound  consists  of  i  atom  of  the 
element  and  2  atoms  of  oxygen  ;  the  second  of  i  atom  of 
the  element  and  3  atoms  of  oxygen,  etc% 

Something  more  was  needed,  how- 

Bring  Disfavor.         ever'  than  mere  arbitrary  rules-    In 
fact,  the  atomic  theory  itself  being 

an  assumption,  further  steps  in  its  development  and 
utilization  should  so  far  as  possible  be  based  on  facts  and 
not  on  other  assumptions.  Such  arbitrary  measures  as 
those  just  described  left  the  whole  matter  in  the  position 
of  '  an  hypothesis  bristling  with  other  hypotheses.' 
Another  inconsistency  of  Dalton,  which  brought  his  en- 
tire theory  into  question  once  more,  lay  in  his  use  of  the 
word  atom.  This  term  covered  both  simple  particles  and 


ATOMIC   THEORY   OF   CHEMISTRY.  Ill 

compound,  the  divisible  and  the  indivisible.  It  was  thor- 
oughly illogical  and  speedily  led  into  difficulties.  It  was 
doubtless  this  confusion  of  ideas  which  led  Dalton  to  reject 
the  relations  of  gaseous  volumes  discovered  by  Gay-Lussac. 

To  avoid  these  difficulties  and  in- 

tedTor  Atoms!""        consistencies,  Davy  first  suggested 

the  use  of  the  word  'proportions' 

instead  of  atomic  weights.  Wollaston  preferred  the  term 
'equivalents,'  formerly  used  by  Cavendish,  and  a  great 
many  followed  his  lead.  In  his  table  of  1814,  Wollaston 
gave  the  combining  weights  of  elements  and  compounds 
together,  calling  all  equivalents  and  declining  to  consider 
them  atomic  weights. 

The  term  'equivalent'  strictly  means  the  weight  of  an 
element  found  by  analysis  of  compounds  which  is  equiva- 
lent to  the  unit  weight  of  the  standard  element  and  will 
combine  with  it  or  with  equivalents  of  other  elements. 
It  differed  in  the  minds  of  Wollaston  and  those  who  fol- 
lowed him  from  the  term  atom  in  that  there  was  no  effort 
whatever  at  settling  the  number  of  supposed  atoms  in  the 
compound  but  the  weights  were  taken  as  found  in  the 
analysis.  Of  course,  if  the  number  of  particles  in  the 
compound  be  considered,  then  the  term  equivalent  be- 
comes identical  with  atomic  weight,  and  unless  they  are 
considered  one  has  in  many  cases  the  choice  between 
several  possible  equivalents.  If  there  had  only  been  a 
few  compounds  to  deal  with  the  matter  would  have  been 
comparatively  simple  but  the  number  was  very  large  and 
was  being  continually  added  to,  so  perplexity  in  the  mat- 
ter of  choice  was  correspondingly  great. 

For  more  than  half  a  century  afterwards,  these  terms 
'combining  weights,'  'proportions,'  and  'equivalents' 
were  used  by  many  very  conservative  chemists  in  prefer- 


112  A   STUDY  OF  THE   ATOMS. 

ence  to  the  term  'atomic  weight' .  Of  course  this  substi- 
tution practically  abandoned  the  idea  of  atoms  and,  in 
theory,  was  but  little  in  advance  of  the  position  held  by 
Richter  and  others. 

Thomson  says  with  regard  to  this  i1  "But  in  fact  these 
terms  'proportion,'  'equivalent'  are  neither  of  them  so 
convenient  as  the  term  atom  ;  and  unless  we  adopt  the 
hypothesis  with  which  Dalton  set  out.  namely,  that  the 
ultimate  particles  of  bodies  are  atoms  incapable  of  further 
division,  and  that  chemical  combination  consists  in  the 
union  of  these  atoms  with  each  other,  we  lose  all  the 
new  light  which  the  atomic  theory  throws  upon  chem- 
istry and  bring  our  notions  back  to  the  obscurity  of  the 
days  of  Bergman  and  of  Berthollet. ' ' 

With  the  discoveries  of  Gay-Lussac,  an- 

Other  mode  °f  determininS  these  combi- 
ning numbers  was  put  into  practice,  and 
that  was  by  a  consideration  of  the  combining  gaseous  vol- 
umes. Not  all  of  the  elementary  numbers  could  be  deter- 
mined in  this  way,  still  there  arose  a  "theory  of  volumes" 
in  which  the  effort  was  made  to  extend  the  idea  theoreti- 
cally to  all  elements.  Thus  they  spoke  of  elementary 
volumes  of  carbon  and  other  solid  elements.  In  deducing 
the  elementary  volume  of  carbon,  for  example,  the  forma- 
tion of  carbon  dioxide  was  considered.  Here  two  volumes 
of  oxygen  are  required  for  the  formation  of  two  volumes 
of  carbon  dioxide.  Now  do  these  contain  one  volume  of 
carbon  or  two  volumes  of  carbon?  Berzelius  decided  from 
analogy  to  the  condensation  in  the  case  of  water  that  there 
was  one  volume  of  carbon.  And  so  we  see  that  here,  too, 
the  old  difficulty  appeared  and  had  to  be  met  by  a  selec- 
tive use  of  analysis  and  hypothesis  and  was  full  of  un- 
certainties. And  yet  many  accepted  this  hypothesis, 

1  Thomson  :  "History  of  Chemistry,"  II.,  p.  294. 


ATOMIC   THEORY   OF   CHEMISTRY.  113 

especially  among  the  French  chemists,  and  sought  to  sub- 
stitute the  word  "  volumes"  for  atoms,  thinking  that  this 
was  more  in  accordance  with  the  facts  and  depended  less 
upon  speculative  hypotheses.  But  after  all,  this,  like  the 
others,  was  nothing  more  than  a  change  of  terms.  In 
1818,  Berzelius  endeavored,  by  formulating  what  he  called 
a  corpuscular  theory,  to  reconcile  the  atomic  theory  of 
Dalton  where  the  fixed  proportions  were  determined  by 
weight,  and  the  elementary  volume  theory,  where  they 
were  found  by  the  combination  of  gaseous  volumes.  He 
spoke  of  indivisible  corpuscles,  ultimate  particles,  chemi- 
cal equivalents,  combining  proportions,  and  molecules  as 
synonymous  with  atoms.  It  is  needless  to  say  that  there 
could  only  be  confusion  of  ideas  where  such  confusion  of 
terms  existed.  He  observed  that  the  atomic  theory, 
theory  of  volumes  and  corpuscular  theory  led  to  about  the 
same  results.  He  came  to  the  conclusion  that  equal  vol- 
umes of  gases  contained  equal  numbers  of  atoms,  but  that 
this  did  not  apply  to  compound  gases.  This  was  an  un- 
fortunate divergence,  as  will  be  seen,  from  the  theory  of 
Avogadro  which  was  at  that  time  practically  ignored,  at 
least  in  its  original  form.  Proust  also,  as  Berzelius  points 
out,  made  use  of  the  volume  theory. 

Thus,  a  dozen  years  after  the  announce- 

Co°f"fi<?n.  ment   of  the  atomic  theory  we  find 

and  Division.  .    .  ,  ,.  .  .      J.      .   . 

great  confusion  and  division  of  opinion; 

Dalton  and  Gay-L,ussac  would  not  accept  the  views  of 
Berzelius ;  Wollaston  rejected  atoms  for  equivalents  ; 
Davy  for  proportional  numbers  ;  the  French  chemists  for 
elementary  volumes  ;  all  with  the  idea  that  they  were 
eschewing  theory  and  confining  themselves  strictly  to 
facts.  Misconception  and  confusion  were  in  a  fair  way, 
as  Wurtz  has  said,  of  rendering  sterile  Dalton' s  profound 
conception  and  consigning  it  to  oblivion. 


CHAPTER  IV. 


The  Relative  Weights  of  the  Atoms. 


CHAPTER  IV. 
THE  RELATIVE  WEIGHTS  OF  THE  ATOMS. 

The  first  and  most  important  development  of  the  atomic 
theory  centers  around  the  determination  of  the  num- 
ber of  atoms  in  the  molecule.  This  problem,  as  has  been 
seen,  formed  a  serious  obstacle  in  the  path  of  chemists  from 
the  very  beginning  of  the  application  of  the  atomic  theory 
and  threatened  to  wreck  the  entire  theory,  though  such 
a  conclusion  was  both  unnecessary  and  illogical.  The 
empirical  rules  of  Dalton  and  especially  of  Berzelius, 
whose  experience  was  much  wider  and  analytical  skill 
much  greater,  gave  very  fair  results  but  there  was  no 
means  of  testing  the  accuracy  attained  and,  if  empiricism 
was  to  be  the  guide,  many  scientific  men  preferred 
pure  empiricism  unmixed  with  theory. 

The  easiest  line  of  attack  of  this 


gaseous  molecules  and  the  first 
generalization  in  this  direction  was  the  theory  of 
Avogadro,  sometimes  called  the  L,aw  of  Avogadro. 
This  theory  was  based  upon  and  offered  in  explanation 
of  three  observed  laws.  First  there  was  Boyle's  law  as 
to  the  effect  of  pressure  upon  volumes  of  gases.  Equal 
volumes  of  gases  were  found  by  Boyle  to  suffer  the  same 
decrease  in  volume  when  subjected  to  equal  pressures  and 
this  was  independent  of  the  nature  of  the  gas.  The  vol- 
ume of  a  gas  was  then  inversely  proportional  to  the  pres- 
sure if  the  temperature  remained  the  same.  Mario  tte 
reached  the  same  conclusion  independently  of  Boyle  some 
seventeen  years  later.  This  law,  announced  in  the  lyth 
century,  has  been  subjected  to  very  careful  testing  in  the 


Il8  A  STUDY  OF  THE  ATOMS. 

1 9th  century.  In  1825  Despretz  showed  that  the  law  was 
not  rigorously  exact.  It  is  a  very  close  approximation 
to  the  truth,  however,  except  for  gases  near  their  points 
of  liquefaction.  Later  experiments  of  Regnault  show 
that  Boyle's  law  is  not  even  true  for  the  more  difficultly 
liquefiable  gases.  It  would  seem  that  there  is  a  tempera- 
ture at  which  the  compressibility  is  exactly  represented 
by  Boyle's  law.  These  facts  were  unknown  at  the  time 
Avogadro  announced  his  theory  (1811),  the  law  being 
then  regarded  as  rigorously  exact. 

It  has  already  been  stated  that  some 
t*w  °*  of  Dalton's  earliest  work  was  upon 

the  effect  of  temperature  upon  the 
volumes  of  various  gases.  In  this  he  anticipated  Gay 
Lusaac.  The  result  of  the  work  of  these  two  investiga- 
tors was  the  establishment  of  the  law  of  temperatures, 
namely,  that  all  gases  expand  alike  for  the  same  increase 
of  temperature.  Hence,  under  constant  pressure  the  vol- 
umes of  gases  are  directly  proportional  to  the  tempera- 
ture. The  coefficient  of  expansion  is  independent  of  the 

pressure,   and  is  now  known  to  be  of  the  volume 

at  o°  for  every  i°  centigrade  between  o°  and  100°. 
This  law,  like  the  previous  one,  was  for  some  time  held 
to  be  strictly  true.  That  it  is  subject  to  the  same  modi- 
fications as  the  law  of  pressures,  has  been  shown  by  the 
experiments  of  Pouillet,  Rydberg,  Magnus  and  Regnault. 
The  coefficient  of  expansion  is  sensibly  affected  by  the 
pressure,  especially  when  gases  near  their  points  of  lique- 
faction, and  this  coefficient  varies  slightly  for  various 
pees,  so  that  it  may  be  said  that  each  gas  has  its  own  co- 
efficient of  expansion  by  heat  as  it  has  its  coefficient  of  com- 
pressibility. In  the  case  of  air,  hydrogen  and  the  more 
permanent  gases,  these  coefficients  approximate  very 
closely  to  one  another. 


RELATIVE   WEIGHTS  OF  THE  ATOMS. 


119 


Law  of 
Volumes. 


The  third  law  is  that  called  the  law  of  vol- 
umes, and  it  was  chiefly  in  explanation 
of  this  that  Avogadro  offered  his  theory. 
This  la w  is  generally  accredited  to  Gay-Lussac,  and  rightly 
so,  as  it  was  established  mainly  through  his  work.  At 
the  beginning  of  the  igih  century,  Gay-Lussac  was  at  work 
upon  the  combination  of  gases  by  volumes.  In  1805, 
working  conjointly  with  Humboldt,1  he  found  that  i  vol- 
ume of  oxygen  and  2  volumes  of  hydrogen  combined  to 
form  water.  They  were  struck  by  the  exactness  of  these 
proportions,  and  further,  that  they  held  good  for  any  tem- 
perature. Gay-Lussac  extended  the  investigation  to 
various  other  gases,  and  in  1808  stated  his  results  before 
the  Socie'te'  Philomathique  in  Paris.  Briefly  summed  up, 
the  law  of  Gay-Lussac  is  that  the  volumes  of  combining 
gases  bear  a  simple  relation  to  each  other,  and  secondly 
that  there  is  also  a  simple  ratio  between  the  volumes  of 
the  gaseous  product  and  of  the  combining  constituents. 
In  stating  his  discovery  Gay-Lussac  recalled  the  discus- 
sion of  Proust  and  Berthollet  over  the  law  of  definite  pro- 
portions and  the  doctrine  of  Dalton  that  substances  com- 
bine by  simple  atoms,  evidently  holding  some  such  theory 
as  the  explanation  of  his  law. 

So  many  examples  were  brought  forward  by  Gay-Lussac 
in  which  the  simple  ratio  of  the  combining  volumes  was 
observable  that  the  generalization  was  soon  accepted.  A 
few  instances,  for  purposes  of  illustration  may  be  men- 
tioned here : 

i  vol.  nitrogen  and  I  vol.  oxygen  give  2  vol.  nitrogen  dioxide. 

hydrochloric  acid. 

water. 

nitrogen  monoxide. 

ammonia. 

ethylene  chloride. 

.  de  Physique,  60,  129. 


i    "    chlorine 

i 

hydrogen  ' 

2     ' 

2    "    hydrogen 

i 

oxygen 

2     ' 

2    "    nitrogen 

i 

oxygen       ' 

2     * 

i    "    nitrogen 

3 

hydrogen   4 

2     ' 

i    "    ethylene 

i 

chlorine     ' 

I     ' 

120  A  STUDY   OP   THE   ATOMS. 

In  the  second  part  of  his  ' '  New  System 
of  Chemistry  "  appearing  in  1810,  Dalton 
spoke  of  the  conclusions  of  Gay-L,ussac  as 
erroneous.  This  was  a  strange  position ,  as  Kopp  remarks , l 
for  one  to  take  with  regard  to  regularity  in  combination 
by  volume  who  had  maintained  the  existence  of  a  similar 
regularity  in  composition  by  weight.  Dalton  stated  that, 
if  it  were  true  that  gases  combined  by  volumes  and  in  so 
simple  a  relation  as  i  with  i,  or  2,  or  3,  then  this  would 
chime  in  well  with  his  theory  of  atoms.  It  was  clear, 
however,  that  it  could  only  be  true  if  equal  volumes  of 
gases  contained  either  the  same  number  of  atoms  or  such 
numbers  as  stand  in  a  simple  ratio  to  one  another.  He 
then  strove  to  show  that  Gay-L,ussac's  generalization  was 
not  supported  by  the  facts.  According  to  his  belief  the 
combination  was  never  exactly  by  equal  volumes.  The 
nearest  approach  to  such  a  regularity  was  to  be  seen  in 
the  combination  of  oxygen  and  hydrogen  to  form  water, 
but  even  here,  according  to  his  most  trusted  experiments, 
it  was  one  volume  of  oxygen  combining  with  1.97 
volumes  of  hydrogen.  He  had  in  former  years  held  that 
the  atoms  of  all  gaseous  bodies  had  the  same  volume  and 
that  in  equal  volumes  of  oxygen  and  hydrogen  there 
were  equal  numbers  of  atoms,  but  on  further  consideration 
he  had  come  to  the  conclusion  that  the  atoms  of  different 
gases  were  not  equally  large. 

Dalton' s  objections  did  not  prevent  the  general  accep- 
tance of  Gay-Lussac's  law.  Careful  workers  speedily 
recognized  its  correctness  within  the  limits  of  experi- 
mental error,  and  that  it  was  not  an  hypothesis,  as  Dalton 
had  called  it,  but  a  generalization  which  could  be  shown 
to  be  true. 

1  "Entwick.  d.  Chem.,"  p.  340. 


RELATIVE   WEIGHTS   OF  THE   ATOMS.  121 

In  spite  of  Dal  ton's  views,  this  fact  of  the 
Law  o  combination  of  gases  by  simple  volume 

gave  strong  support  to  the  atomic  theory. 
It  fell  into  line  with  the  observation  of  the  fixed  relation 
by  weight  of  the  combining  bodies,  and  the  multiple 
weight  relations  are  encountered  again  in  the  combination 
by  volumes.  If  this  law  of  volumes  is  true  then  there 
should  exist  a  simple  relation  also  between  the  specific 
gravities  of  elementary  gases  and  their  atomic  weights. 
This  was  seen  by  Gay-Lussac  and  clearly  shown  and  de- 
fined by  Berzelius,  but  Dalton  refused  to  accept  it  and  ig- 
nored this  also.  There  was  some  force  in  Dalton' s  ob- 
jection. The  relation  between  the  atomic  weights  and 
specific  gravities  was  not  so  simple  as  had  seemed  at  first 
sight,  though  for  quite  a  while  it  was  held  to  be  simple. 
It  has  required  half  a  century  to  remove  all  of  the  diffi- 
culties. The  following  table  gives  the  relations  for  some 
of  the  elements.  In  this  table  the  numbers  in  the  second 
column  represent  the  densities  (W),  or  specific  gravities 
of  the  elements  in  the  form  of  gas,  in  the  third  column 
are  the  atomic  weights  (w) ,  and  in  the  fourth  column 
the  ratios  (w\d)  between  the  atomic  weights  and  the  den- 
sities. 

d.  w. 

Element.  Density.  At.  wt.  w\d. 

Hydrogen 0.0692  i  .o  14-45 

Chlorine 2.440  35.4  I4-5I 

Bromine 5.54  79.9  14.42 

Iodine 8.716  126.5  14.51 

Oxygen 1 .10563  8.0  7.24 

Sulphur 2.23  16.0  7.17 

Nitrogen 0.9713  14.0  14.41 

Phosphorus 4.50  31.0  6.89 

Mercury    7.03  99.9  14.21 

Cadmium 3.94  55.9  14.19 

In  the  case  of  seven    of  these    elements    the   rela- 


122  A   STUDY   OF   THE   ATOMS. 

tion  between  densities  and  the  atomic  weights  is  prac- 
tically the  same,  averaging  14.4.  For  the  remaining  ele- 
ments it  is  also  virtually  the  same  but  much  smaller  than 
in  the  preceding  case,  the  number  7.1  being  about  one- 
half  the  former  number. 

Another,  and  more  usual,  mode  of  stating  this  relation  is 
that  the  atomic  weights  are  proportional  to  the  densities. 
Thus: 

M  :  M'  :  :  d  :  <T 

where  d  and  <T  are  the  densities  and  M,  M'  represent  [the 
atomic  weights,  or  in  the  further  extension  of  the  law 
the  molecular  weights. 

Gathering  together  the  facts  which  we 
Theory  of  faave  beefl  <jiscnssi11g%  we  find  that  the 
Avogadro.  ...  .  „ 

volumes    of    all  gases  vary   alike    with 

changes  of  pressure  and  that  the  same  is  true  for  changes 
of  temperature.  Again  the  relation  between  the  combin- 
ing volumes  of  gases  is  a  simple  one  and  the  specific 
gravities  of  the  elementary  gases  are  proportional  to  their 
atomic  weights.  There  would  seem  to  be  but  one  ex- 
planation of  these  facts,  one  cause  underlying  them  all, 
if  the  atomic  theory  is  true.  Dalton  saw  the  necessary 
deduction  and  stated  it  but  challenged  the  truth  of  the 
facts.  These  equal  volumes  of  gases  must  contain  the 
same  number  of  ultimate  particles.  This  was  re-stated  a 
year  later,  in  1811,  by  Amadeo  Avogadro  and  it  is  gener- 
ally known  as  Avogadro' s  Theory  though  Debus  would 
call  it,  and  with  some  justice,  the  Dalton-Avogadro 
theory.  Three  years  later,  in  1814,  Ampere  also  an- 
nounced the  same  conclusion  as  having  been  reached  by 
him,  being  in  ignorance  of  the  writers  who  had  preceded 
"Him  In  fact,  so  simple  is  the  deduction  that  it  would 
seem  strange  if  many  had  not  clearly  drawn  it.  And  yet 


RELATIVE  WEIGHTS   OF  THE   ATOMS.  123 

the  support  it  gave  to  the  atomic  theory  and  its  great  im- 
portance were  not  duly  recognized.  The  ultimate  particles 
of  the  theory  were  not  of  necessity  atoms  but  might  be  any 
minute  portions,  divisible  or  indivisible.  Some  accepted 
the  law  of  volumes  and  sought  to  substitute  it  for  the 
atomic  theory.  Berzelius,  and  others  under  his  lead, 
caught  the  idea  at  first  and  made  use  of  it,  but  misinter- 
preting, or  rather  misstating  the  theory,  speedily  en- 
countered difficulties  and  contradictions  and  the  theory 
was  relegated  to  a  subordinate  place  with  them.  Dal  ton 
rejected  the  idea  because  of  exceptions  and  inconsistencies 
which  he  could  not  explain  and  thereby  lost  his  claim  to 
part  in  the  theory.  It  was  not  until  1858  that  Canniz- 
zaro  insisted  upon  the  immense  importance  of  this  theory 
and  made  use  of  it  to  rescue  the  physical  constants  of 
chemistry  from  a  state  of  unhappy  confusion. 

A  prime  obstacle  in  the  way 
22S3  SEL.        of theacceptanceof  theth^ 

of  Avogadro  lay  in  the  lack  of 

a  clear  distinction  between  the  varieties  of  ultimate  parti- 
cles. It  was  known  that  these  were  of  two  kinds,  parti- 
cles of  elements  and  particles  of  compounds,  but  they  were 
not  distinguished  from  one  another,  and  speaking  of  and 
treating  them  alike  necessarily  bred  confusion  of  ideas. 
The  word  atom  was  used  interchangeably  for  both  kinds 
of  particles  and  hence  did  not  mean  always  the  simple 
indivisible  ultimate  particle.  With  that  idea  fixed,  the 
other  particles  made  up  of  atoms,  whether  in  an  elemen- 
tary gas  or  in  a  compound,  would  have  been  clearly  differ- 
entiated. 

Avogadro  had  made  a  clear  distinction  and  suggested 
two  names.  When  a  substance  splits  up  in  its  conversion 
into  the  gaseous  state  it  is  divided  into  a  number  of  small- 


124  A   STUDY  OF   THE   ATOMS. 

est  particles  which  he  called  molecules  integrantes  or  con- 
stituantes.  These  he  defined  as  particles  of  matter  which 
were  so  far  apart  that  there  was  no  longer  any  mutual  at- 
traction exerted  and  they  were  merely  subject  to  the  re- 
pelling action  of  heat.  These  particles  he  regarded  as 
groups  of  several  individual  atoms  (molecules  elementaires} 
united  by  mutual  attraction.  This  is,  of  course,  in  part  the 
present  distinction  which  is  drawn  between  molecules  and 
atoms  and  is  indispensable  in  all  chemical  theories.  It 
would  have  been  of  immense  service  if  the  suggestion  of 
Avogadro  had  been  followed,  but  it  seems  to  have  received 
scant  notice.  It  is  difficult  to  account  for  the  blindness 
of  such  leaders  as  Berzelius  and  Dal  ton  and  Davy,  except 
on  the  ground  that  it  was  at  a  formative  period  of  the 
science  and  the  general  conditions  chaotic. 

The  distinction  was  absolutely  es- 

sential  for  the  truth  of  Avogadro's 
theory.  If  his  ultimate  particles 
were  atoms,  then  the  theory  failed  to  hold  good  in  a  num- 
ber of  cases.  If  they  were  compound,  or  molecules,  then 
the  explanation  served.  Thus  experiment  shows  that  i 
volume  of  oxygen  and  2  volumes  of  hydrogen  unite  to 
form  2  volumes  of  water.  But  if  each  volume  had  an 
equal  number  of  atoms  and  i  oxygen  atom  was  in  each 
particle  of  water,  it  would  manifestly  be  impossible  that 
there  should  be  formed  more  than  i  volume  of  water. 
Again,  i  volume  of  hydrogen  and  i  volume  of  chlorine 
produced  2  volumes  of  hydrochloric  acid,  but  the  same 
reasoning  would  show  that  there  could  not  be  more  than 
i  volume  of  hydrochloric  acid.  It  was  from  such  reason- 
ing as  this  that  Dal  ton  rejected  the  hypothesis  of  the  ex- 
istence of  equal  numbers  of  particles  in  each  volume.  If 
now,  it  is  assumed  that  each  particle  of  oxygen,  of  hydro- 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  125 

gen  and  of  chlorine  was  not  an  atom  but  a  molecule  con- 
taining 2  atoms,  then  the  theory  that  the  volumes  con- 
tained equal  numbers  of  particles  becomes  entirely  prob- 
able and  accords  with  the  belief  in  combination  by  atoms 
as  well  as  accounts  for  the  relation  between  the  specific 
gravities  and  the  atomic  weights.  Avogadro's  statement 
of  the  case  was  not  worded  as  that  above.  He  said  that  a 
molecule  of  water  is  made  up  of  a  half-molecule  of  oxygen 
and  2  half-molecules,  or  i  whole  molecule,  of  hydrogen. 
He  did  not  expressly  state  what  relations  the  particles, 
which  he  calls  molecules,  bear  to  the  atomic  weights,  but 
he  made  it  clear  that  he  considered  the  combining  weights 
only  fractions,  as  a  rule,  of  the  molecules.  It  is  evi- 
dent that  his  mode  of  expression  was  not  clearly  under- 
stood, as  Berzelius1  in  1826  declared  the  views  of  Avo- 
gadro  absurd  since  he  sought  to  divide  the  atoms  which 
were  indivisible. 

As  to  these  particles,  Davy2  had  espoused  the  view  that 
the  atoms  first  combine  to  form  regularly  arranged  groups 
and  then  these  unite,  like  elementary  particles,  to  form 
various  bodies.  Ampere  in  1814*  advanced  opinions  sim- 
ilar to  those  of  Avogadro.  He  also  aimed  at  gaining  some 
conceptions  of  the  number  and  arrangement  of  the  ele- 
mentary atoms  which  form  the  molecules  of  different  sub- 
stances. The  particles  which  he  had  in  mind,  however, 
were  those  which  go  to  form  crystalline  bodies.  Hauy 
used  for  these  the  name  molecules  integrantes.  Avogadro, 
however,  did  not  look  with  favor  upon  this  extension  of 
his  theory.  Avogadro's  own  efforts  at  extending  his 
theory  to  cases  where  no  observation  of  the  density  in 
the  state  of  a  gas  had  been  made,  or  could  be  made,  were 

i  Jahresbericht,  1826,  p.  80. 

9  Davy  :  "  Elements  of  Chemical  Philosophy,"  1812. 

8  Annal.  dt  Chimie,  90,  43. 


126  A   STUDY   OP  THE   ATOMS. 

also  unfortunate  and  tended  to  bring  discredit  upon  the 
theory. 

Gradually  confirmation  of  the  truth 

of  the  theory  of  Av°Sadro  came  f rom 
various  sides.     Schroeder1   based   a 

certain  argument  in  its  support  upon  the  physical  proper- 
ties of  bodies,  in  particular  the  boiling-points  of  chemical 
compounds.  Clausius3  recognized  the  necessity  for  the 
theory  from  physical  reasons  arising  from  the 
mechanical  theory  of  heat.  Its  acceptation  by  chemists, 
as  Lothar  Meyer  says,3  was  because  the  molecular  weights 
determined  by  its  aid  appeared  the  only  numbers  capable 
of  serving  as  the  basis  of  a  theoretical  speculation  on  all 
the  different  chemical  transformations,  and  especially  be- 
cause this  hypothesis  established  an  analogy  between  the 
so-called  elements  and  their  compounds  by  regarding 
the  former  as  compounds  of  similar  atoms  and  the  latter 
as  compounds  of  dissimilar  atoms. 

Several  facts  can  be  adduced  to  support  the 
assumption  that  the  ultimate  particles  of 
elementary  gases  are  composed  of  atoms  and 
are  not  single  atoms.  An  argument  can  be  drawn  from 
the  chemical  fact  that  some  of  these  gases  behave  very 
differently  when  they  are  just  being  liberated  from  com- 
pounds and  when  they  are  in  the  ordinary  gaseous  con- 
dition. In  the  first  case  they  are  said  to  be  in  the  nascent 
state  and  they  show  far  greater  chemical  activity.  Thus 
hydrogen  in  its  usual  condition  shows  little  tendency 
to  combine  with  most  other  elements  and  compounds.  On 
the  contrary,  if  it  is  just  being  liberated  from  some  pre- 
existing compound  it  is  capable  of  uniting  immediately 
with  a  number  of  bodies  and  of  bringing  about  very 

1  "Die  Siedhtze  d.  Chem.  Verb."  (1844),  27,  67, 138. 

*  Pogg.  Annalen,  100,  369  (1857);  103,  645  (1858). 

»  "  Modern  Theories  of  Chem."  (Eng.  trans.),  1888,  p.  12. 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  127 

material  changes  in  bodies  with  which  it  comes  in  con- 
tact. Similar  facts  have  been  observed  with  regard  to 
oxygen,  nitrogen  and  other  elements.  The  only  plausible 
explanation  which  has  been  advanced  to  explain  these 
facts  is  that  in  the  ordinary  condition  of  these  gases  one 
is  dealing  with  molecules  consisting  of  at  least  two  atoms, 
thus :  HH,  OO,  NN,  etc.  As  these  atoms  are  already 
united  with  one  another  there  is  little  or  no  tendency  to 
effect  a  material  change  on  any  other  body  until  they 
have  been  separated  and  so  are  free  from  the  formation 
of  fresh  compounds.  If  the  element  is  j  ust  being  liberated 
from  some  compound,  as  when  nitric  acid  acts  upon  zinc, 

Zn  +  2HNOs  =  Zn(NO3)2  +  Hs, 

then  it  may  be  assumed  that  each  atom  of  hydrogen 
when  set  free  from  the  molecule  of  nitric  acid  remains 
uncombined  for  a  brief  fraction  of  time  and  in  this  condi- 
tion shows  a  greatly  increased  tendency  to  form  com- 
pounds. If  it  meets  with  some  other  element,  or  group 
of  elements,  with  which  it  can  combine,  immediate  com- 
bination takes  place.  If  no  such  body  is  present,  then  it 
combines  with  another  hydrogen  atom  and  thus  forms  a 
molecule  made  up  of  similar  atoms.  It  must  be  borne  in 
mind  that  there  are  no  proofs  of  this.  It  is  merely  an 
hypothesis  to  explain  the  fact  of  greater  chemical  activity 
being  exibited  in  one  case  than  in  the  other.  An  argu- 
ment then  which  involves  so  much  assumption  should 
not  have  too  much  reliance  placed  upon  it.  Some 
chemists  contend  that  the  hypothesis  of  a  nascent  state 
should  be  discarded  altogether  as  needless,  but  so  far, 
nothing  more  satisfactory  has  been  suggested  to  explain 
the  facts. 

o  To  show  that  the  simple  hypothesis  is  not  al- 

ways satisfactory  or  accepted,  it  may  be  men- 
tioned that  there  is  a  very  active  form  of  oxygen,  called 


128  A   STUDY   OF   THE   ATOMS. 

ozone,  which  is  formed  in  many  reactions  when  oxygen 
is  liberated  from  certain  compounds,  as  for  instance  from 
carbon  dioxide,  or  when  this  element  is  acted  upon  by 
electricity,  and  in  a  number  of  other  ways.  This  active 
form  of  oxygen,  however,  is  not  called  nascent  oxygen 
nor  is  it  supposed  to  consist  of  single  separate  atoms. 
The  fact  that  it  exists  free  for  appreciable  lengths  of 
time,  of  course,  differentiates  it  from  the  ordinary  nascent 
elements.  Considerations  of  density  have  led  chemists 
to  conclude  that  in  this  case  the  molecule  consists  of  three 
atoms.  They  then  attribute  the  great  energy  exhibited  by 
it  to  its  instability  and  the  ease  with  which  it  breaks  up 
into  two-atomed,  or  the  ordinary  oxygen,  and  a  single- 
atomed,  or  nascent  oxygen,  which  is  supposed  to  be  the 
energetic  portion.  There  is  much  in  the  behavior  of  the 
body  to  lend  strength  to  these  suppositions. 

This  instance  is  cited  here  merely  to  show  the  amount 
of  assumption  involved  and  the  doubt  which  can  be  thrown 
upon  all  arguments  from  a  presumed  nascent  state.  It 
will  be  observed  that  in  an  argument  intended  to  strengthen 
the  Avogadro  theory,  this  theory  itself  is  appealed  to  in 
order  to  establish  the  presence  of  3  atoms  in  the  molecule 
of  ozone,  ordinary  oxygen  being  assumed  to  have  2  atoms 
to  the  molecule,  and  in  the  decomposition  of  ozone  the 
single  atom  set  free  is  said  to  exercise  the  powerful 
oxidizing  action, — all  of  which  is  plausible  and  may  be 
true,  but  certainly  is  without  direct  proof.  Since  there  is 
entire  ignorance  as  to  the  density  of  active  hydrogen  or 
active  nitrogen  it  is  manifestly  a  possible  assumption  that 
these  also  are  molecules  of  two  or  more  atoms.  At 
bottom  the  facts  are  that  various  gaseous  elements  exist 
in  two  or  more  forms  which  differ  in  physical  properties 
and  in  chemical  activity.  This  existence  of  an  element 


RELATIVE   WEIGHTS   OF  THE   ATOMS.  I2Q 

in  two  or  more  different  forms  is  known  as  allotropism 
and  may  be  observed  in  the  liquid  and  solid  as  well  as  the 
gaseous  states.  The  only  plausible  explanation  of  this 
phenomenon  which  has  been  offered  involves  the  atomic 
hypothesis  and  assumes  that  the  ultimate  particles  of 
these  elements  are  in  the  different  cases  made  up  of  differ- 
ent numbers  of  atoms. 

Returning  now  to  the  hypothesis  that 
lnte™°J®cu"  the  particles  in  elementary  gases  are 

really  molecules  made  up  of  at  least  two 
atoms  there  are  certain  physical  facts  which  may  be  ad- 
duced to  confirm  this  belief.  In  bringing  forward  these 
arguments  the  kinetic  theory  of  gases  is  supposed  to  be 
true.  The  total  energy  of  the  molecules  of  a  gas  rep- 
resents the  amount  of  heat  absorbed  by  the  gas.  When 
the  molecule  is  looked  upon  as  a  material  point  this  energy 
can  only  be  progressive  motion.  Taking  this,  it  is  not 
difficult  to  calculate  the  relation  between  the  specific  heat 
of  a  gas  at  constant  volume  and  that  at  constant  pressure. 

Let  h  and  h'.  represent  these  two  specific  heats  respec- 

1 1 
tively,  then  this  ratio  is  -7-  =  c.    According  to  theory  for 

a  material  point  without  moving  parts,  c  =  1.67.  In  the 
case  of  most  gases  examined  the  observed  value  is  less  than 
this  theoretical  one,  c  being  equal  to  1.405.  Thus  it  is 
seen  that  when  the  volume  remains  unchanged  more  heat 
is  actually  required  to  raise  the  temperature  of  a  gas  than 
the  theory  demands.  A  portion  of  the  heat  therefore 
disappears  and  it  may  be  that  this  is  transformed  into 
motion  between  the  atoms  which  compose  the  molecule, 
or  what  is  called  intramolecular  work.  If  there  is  but 
one  atom  and  so  no  intramolecular  motion  possible,  then 
the  ratio  observed  should  be  the  same  as  the  theoretical, 


130  A  STUDY  OF  THE  ATOMS. 

namely  1.67.  This  ratio  has  been  observed  in  the  case  of 
a  few  gases  only,  as  gaseous  mercury,  cadmium  and 
possibly  other  metals,  also  argon,  helium,  etc.  These 
may  be  assumed  to  have  molecules  of  one  atom.  In  the 
others  there  would  seem  to  be  a  greater  number  of  atoms. 
Again  Kundt  and  Warburg1  have  made  use  of  the 
propagation  of  sound  in  gases.  Here  also  evidence  can 
be  gotten  as  to  internal  motion  in  the  particles  and  the 
evidence  accords  with  that  secured  by  the  method  just 
mentioned.  Thus  mercury  vapor  in  sound  experiments 
also  acts  as  if  the  particles  were  material  points. 

Enough   then   is  known   to  make  it 

o/the  ™eorC  .  very  Probable  that  Sases  consist  of 
compound  particles  and  rarely  of  in- 
divisible atoms.  In  using  Avogadro's  theory  in  the 
determination  of  atomic  weights  it  becomes  necessary  to 
assume  one  molecular  weight  as  known.  Since  the  molecu- 
lar weights  are  proportional  to  the  densities  we  have 

m  :  m'  :  :  d  :  df 

where  m  and  mf  are  the  molecular  weights  and  d  and  d' 
are  the  densities.  Then, 

»'=<f  X-^-. 

a 

To  solve  this,  d  and  d'  are  densities  which  can  be  deter- 
mined by  experiment  but  m  cannot  be  so  determined.  Still 
if  one  such  molecular  weight  be  assumed  then  all  others 
can  be  based  upon  it.  Hydrochloric  acid  is  the  gas  whose 
molecular  weight  is  assumed  as  a  standard.  This  com- 
pound contains  by  weight  35.4  parts  of  chlorine  to  i  part 
of  hydrogen.  A  smaller  figure  would  necessitate  repre- 
senting the  atomic  weight  of  hydrogen  as  less  than  unity, 
which  is  of  course  not  an  impossibility  if  that  which  has 

1  Ber.  d.  chem.  Ges.,  1875,  p.  945. 


RELATIVE    WEIGHTS   OF  THE   ATOMS.  131 

hitherto  been  assumed  as  the  atomic  weight  of  hydrogen 
be  in  reality  the  weight  of  more  than  one  atom.  On  the 
other  hand  it  may  be  said  that  no  facts  are  known  that 
require  the  selection  of  a  number  greater  than  36.4  (i  + 
35.4)  for  the  molecular  weight  of  hydrochloric  acid. 
Should  facts  become  known  which  require  a  smaller  figure, 
then  all  of  the  determinations  of  molecular  weights  must 
be  changed  in  proportion.  With  this  assumption  the 
above  equation  becomes 

m'=d'X-&±=d'X  28.57- 
1.247 

This  figure,  28.57,  should  be  the  quotient  obtained  by 
dividing  the  molecular  weight  of  any  known  gas  by  its 
specific  gravity.  Experimental  errors,  however,  cause  a 
slight  variation  from  this.  The  number,  28.88,  is  more 
nearly  the  average  of  many  results. 

By   means  of  this   method   the 

Results  as  to  molecules    of    most    elementary 

Caseous  Molecules. 

gases  have  been  found  to  consist 

of  two  atoms.  This  may  be  deduced  from  the  fact  that 
in  every  case  of  combination  of  elementary  gases,  where 
one  volume  of  one  gas  is  taken  the  resulting  compound 
occupies  two  volumes.  Certain  elements  as  mercury, 
argon,  helium,  etc.,  in  the  form  of  vapor  have  apparently 
only  one  atom  in  the  molecule,  as  has  already  been  men- 
tioned. Others  as  sulphur,  phosphorus,  arsenic,  etc., 
have  molecules  containing  different  numbers  of  atoms 
according  to  the  temperature  to  which  they  are  heated. 
Thus  sulphur  at  860°  has  a  density  of  2.23  which  corre- 
sponds to  two  atoms  in  the  molecule,  at  524°  its  density  is 
three  times  as  great,  which  would  lead  to  the  conclusion 
that  the  molecule  contains  six  atoms.  Another  instruc- 
tive example  is  seen  in  the  case  of  iodine.  The  specific 


132  A   STUDY   OF   THE   ATOMS. 

gravity  of  iodine  vapor  at  low  temperatures  is  8.8  which 
corresponds  to  a  molecular  weight  of  254,  or  a  molecule 
of  two  atoms.  At  1027°  this  density  becomes  5.8,  at  1468° 
it  is  further  reduced  to  5.1.  At  the  highest  temperature 
at  which  the  observations  could  be  carried  out  it  was 
found  to  be  4.5  which  is  very  nearly  half  the  first  specific 
gravity  and  indicates  a  molecule  of  one  atom.  It  has 
not  been  possible  to  go  beyond  this  point.  This  has  been 
taken  as  a  possible  indication  that  at  very  high  tempera- 
tures all  gases  consist  of  molecules  made  up  of  single  atoms. 

A  similar  decomposition  of  the 

Supposed  Exceptions  molecules  of  compound  bodies 
to  the  Theory. 

when  vaporized  at  high  tem- 
peratures has  been  shown  in  several  interesting  cases  by 
vapor-density  determinations,  and  the  dissociation  some- 
times confirmed  by  additional  experimental  observations. 
These  were  at  first  supposed  to  be  exceptions  to  the  work- 
ing of  the  Avogadro  theory.  Ammonium  chloride  can 
be  formed  by  the  combination  of  one  volume  of  ammonia 
and  one  volume  of  hydrochloric  acid.  The  analysis  of  this 
compound  gives  the  proportion  between  the  elements  as 
nitrogen  14,  hydrogen  4,  and  chlorine  35.4,  or  by  atoms 
as  nitrogen  i,  hydrogen  4,  and  chlorine  i.  The  density 
of  a  molecule  of  this  compound,  that  is,  of  as  much  as 
would  occupy  the  same  space  as  a  molecule  of  hydrogen, 
would  be  53.4.  But  the  density  as  determined  by  ex- 
periment is  only  26.69,  or  half  as  much  as  would  be  ex- 
pected. This  would  mean,  if  the  theory  is  correct,  that 
in  ammonium  chloride  the  atoms  of  nitrogen  and  chlorine 
are  only  half  so  large  as  in  all  other  known  compounds 
and  the  formula  would  be  Ni/2H2Cli/2,  or  the  formulas  of  the 
other  compounds  of  nitrogen  and  chlorine  would  have  to 
be  doubled,  nitrogen  being  taken  as  7  and  chlorine  as  17.7 


RELATIVE   WEIGHTS   OF   THE   ATOMS.  133 

and  the  formula  for  ammonium  chloride  being  taken  as 
NH2C1.  Then  the  densities  of  these  other  compounds 
would  show  them  all  to  vary  from  that  required  by  the 
theory.  Unless  some  other  explanation  is  found  the  di- 
lemma is  a  serious  one,  either  ammonium  chloride  is  an 
exception  or  the  other  compounds  are,  and  exceptions  are 
fatal  to  theories.  Some  time  passed  before  a  satisfactory 
explanation  was  found  but  it  can  now  be  shown  that 
when  ammonium  chloride  is  volatilized  the  vapor  is  not 
that  of  the  ammonium  chloride  but  of  the  mixed  gases 
ammonia  and  hydrochloric  acid.  Hence  the  density 

will  be  the  mean  of  these  twof  — —=26.7)  and  not 

that  of  the  two  combined  (53.4)  and  this  accords  with 
the  observed  density. 

Phosphorus  pentachloride,  PC15,  was  also  once  regarded 
as  an  exception  but  a  similar  splitting  up  of  the  molecule 
into  PC13  and  C12  can  be  proved.  In  this  case  the  dis- 
sociation is  a  gradual  one  with  the  rise  of  temperature 
and  hence  varying  densities  are  observed.  Recent  ex- 
periments seem  to  show  that  this  and  the  above 
dissociation  are  due  to  the  presence  of  traces  of 
water,  and  that  they  do  not  take  place  in  the  perfectly 
dry  gas  at  moderately  high  temperatures.1  The  im- 
portant fact  to  be  noted  j  ust  here  is  that  the  method  of 
densities  combined  with  Avogadro's  theory  reveals  this 
dissociation  and  its  extent.  Several  other  cases  are 
known  in  which  the  observed  density  varies  from  the 
theoretical  but  they  are  capable  of  being  explained  in  the 
same  way  as  the  instances  cited  and  indeed  in  nearly  all 
of  these  there  is  convincing  proof  that  such  explanation 
is  the  correct  one.  These  apparent  exceptions  then  are 

1J.  Chem.  Soc.  (I,ondon),  1900,  p.  646. 


134  A   STUDY  OF  THE   ATOMS. 

really  strong  confirmations  of  the  truth  of  the  theory. 
While  this  is  apparent  now  it  must  be  borne  in  mind  that 
for  many  years  no  explanation  of  these  exceptions  was 
known  and  they  tended  greatly  to  discredit  the  theory. 
These  points  will,  however,  be  taken  up  later. 

In  the  year  1819  Dulong  and  Petit 
Specific  Heats.  Polished1  the  specific  heats  of  13 

chemical  elements  adding  the  obser- 
vation that  these  specific  heats  were  as  a  rule  inversely 
proportional  to  the  atomic  weights.  The  consequent 
deduction  from  this  is  that  the  specific  heat  is  directly 
proportional  to  the  number  of  atoms  contained  in  the  unit 
weight.  This  then  gives  a  method  of  determining  the 
number  of  atoms  in  molecules  of  solids.  The  table  given 
by  Dulong  and  Petit  was  as  follows : 

Specific  Relative  weights         Weight  X 

Element.  heat.  of  atoms.  specific  keat. 

Bismuth  ....  0.0288  13.30  0.3830 

Lead 0.0293  12.95  0.3794 

Gold 0.0298  12.43  0.3704 

Platinum 0.0314  11.16  0.3740 

Tin 0.0514  7.35  0.3879 

Silver 0.0557  6. 75  0.3759 

Zinc 0.0927  4.03  0.3736 

Tellurium...  0.0912  4.03  0.3675 

Nickel 0.1035  3.69  0.3819 

Iron o.noo  3.392  0.3731 

Cobalt 0.1498  2.46  0.3685 

Sulphur 0.1880  2.011  0.3780 

As  to  this  table  it  must  be  stated  first  that  there  were 
several  errors  which  were  afterwards  corrected  by  Reg- 
nault.  In  the  case  of  tellurium  and  cobalt  the  specific 
heats  were  too  low.  The  atomic  weights  were  given 
with  oxygen  as  unity.  To  compare  these  with  those  of 
Berzelius  they  must  be  multiplied  by  100.  On  making 

1  Ann.  chim.  phys.,  10,  395-413. 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  135 

the  comparison  it  will  be  seen  that  a  number  of  them,  as 
those  of  zinc,  iron,  nickel,  copper,  lead,  tin,  gold,  and 
tellurium  were  only  half  as  large  as  those  given  by  Ber- 
zelius.  In  choosing  this  smaller  number  Dulong  and 
Petit  were  influenced  by  the  notable  regularity  observed 
in  connection  with  other  atomic  weights.  In  determining 
upon  an  atomic  weight  by  combining  proportions  there 
were  often  two  or  three  numbers  to  choose.between,  which, 
however,  bore  a  very  simple  relation  to  one  another. 
Hence  the  choice  was  always  to  some  extent  arbitrary. 
The  regularity  observed  by  Dulong  and  Petit,  and  called 
by  them  a  law,  might  most  justly  be  used  to  arrive  at  a 
decision  between  such  combining  proportions,  obtained 
by  analysis.  These  authors  remark  that  ' '  the  mere  in- 
spection of  the  numbers  obtained  points  to  a  relation  so 
remarkable  in  its  simplicity  as  to  be  at  once  recognized  as 
a  physical  law,  susceptible  of  being  generalized  and  ex- 
tended to  all  elementary  substances.  In  fact  the  prod- 
ucts in  question  which  express  the  capacities  for  heat 
of  atoms  of  different  natures  are  so  nearly  the  same  for 
all  that  we  cannot  but  attribute  these  very  slight  differ- 
ences to  inevitable  errors,  either  in  the  determination 
of  capacities  for  heat  or  in  the  chemical  analysis. ' '  The 
law  was  further  stated  by  Dulong  and  Petit  as  follows  : 
"  The  atoms  of  all  simple  bodies  have  precisely  the 
same  capacity  for  heat."  While  the  former  mode  of 
statement  bore  especially  upon  the  determination  of 
atomic  weights,  which  was  the  burning  question  of  the 
time,  and  so,  was  the  point  of  view  from  which  the  law 
was  most  commonly  regarded,  the  latter  is  most  sig- 
nificant as  regards  the  atoms  and  their  nature  and  hence 
is  most  important  from  the  standpoint  of  this  present 
study. 


136  A  STUDY  OF  THE  ATOMS. 

As  a  simple  generalization  based 
upon  easily  substantiated  facts 
it  might  nave  been  expected 
that  the  law  of  Dulong  and  Petit  would  have  been  imme- 
diately and  widely  accepted  by  chemists  but  such  was  not 
the  case.  One  cause  for  this  lay  in  the  errors  and  inac- 
curacies mentioned  above,  and  further  in  the  apparent  ex- 
ceptions which  soon  came  under  observation.  Another 
cause  is  to  be  found  in  the  influence  of  Berzelius,  the 
weight  of  which  was  thrown  against  its  immediate  accept- 
ance. He  recognized  the  great  importance  of  the  law  for 
theoretical  chemistry  but  thought  that  some  of  the  atomic 
weights  assumed  by  the  authors  would  give  improbable 
relations  for  the  compounds  of  these  elements.  Thus,  for 
instance,  the  atomic  weights  of  zinc,  iron,  nickel,  copper, 
lead,  tin,  gold,  and  tellurium  would  make  the  oxides  of 
those  metals  monoxides,  ZnO,  FeO,  etc.,  while  Berzelius 
regarded  them  as  dioxides.  It  was  possible,  of  course, 
that  the  relations  hitherto  accepted  for  these  elements, 
reasoned  from  analogy,  did  not  exist.  It  was  possible 
that  the  generalization  of  Dulong  and  Petit  did  not  hold 
in  some  cases  and  to  decide  this  the  investigation  should 
be  extended.  As  he  could  not  arrive  at  a  decision  as  to 
this  he  determined  for  the  time  to  retain  his  former 
atomic  weights.1 

Berzelius  had  expressed  the  desire  for 
Application  th  appijcation  of  tbe  generalization 

to  Compounds. 

of  Dulong   and   Petit   to  compound 

bodies.  This  was  first  done  by  F.  Neumann2  in  the  year 
1831,  who  showed  that  equivalent  quantities  of  compounds 
having  analogous  composition  have  the  same  specific 

1  Berzelius  :  "  Jahresbcricht,"  I,  19,  and  XXI,  6. 

2  Pogg.  Annalen,  33,  i. 


RELATIVE   WEIGHTS   OF  THE   ATOMS.  137 

heats.  This  was  not  due  to  the  bodies  having  the  same 
crystalline  form.  It  was  true  even  when  the  crystalline 
form  differed.  Neumann's  extension  of  the  law  may  be 
stated  in  the  same  form  as  the  law,  namely  for  com- 
pounds of  analogous  composition  the  specific  heats  are 
inversely  proportional  to  the  molecular  weights  of  the 
compounds,  or  the  molecules  of  different  compounds  have 
equal  capacity  for  heat.  The  product  of  the  molecular 
weight  multiplied  by  the  specific  heat  is  then  a  constant 
quantity.  Thus : 

Lead  chloride,  0.0664  X  459.48  =  19.62  ; 
Lead  bromide,  0.0533  X  365.92  =  19.50  ; 
Lead  iodide,      0.0427  X  459.48  =  18.40  ; 
or  again, 

Calcium  chloride,      0.1642  X  110.06  =  18.07  J 
Strontium  chloride,  0.1199  X  158.04—  18.95  J 
Barium  chloride,       0.0902  X  207.64  =  18.73. 
From  a  large  number  of  similar  results  the  deduction 
can  be  made  that  an  element,  whether  in  the  free  state  or 
in  combination,  possesses  the  same  specific  heat.     Thus, 
take  the  case  of  lead  iodide.     If  the  specific  heat  of  lead 
is  multiplied  by  its  atomic  weight  we  have 

0.0307  X  206.4  =  6.34, 
and  so  for  iodine, 

0.0541  X  126.54  =  6.85. 
There  are,  however,  2  atoms  of  iodine  in  lead  iodide,  hence 

6.85  X  2  =  13.70. 

The  specific  heats  of  the  constituents  of  lead  iodide  are 
therefore 

6-34  +  13-79  =  2G>-°4- 

The  specific  heat  of  lead  iodide  determined  by  experiment 
is  19.62  which  agrees  well  with  the  other  and  so  the  above 
deduction  is  justifiable.  From  this  it  is  easy  to  see  how 


138  A   STUDY  OF  THE   ATOMS. 

the  specific  heat  may  be  used  to  determine  the  number  of 
atoms  in  a  molecule  and  so  to  make  it  possible  to  choose 
correctly  between  two  or  more  possible  weights  for  the 
atom  obtained  by  analysis.  The  determination  of  the 
specific  heat  involves  so  many  difficulties  that  the  direct 
determination  of  the  atomic  weight  by  means  of  it  is  only 
an  approximation. 

As  has  been  stated,  the  first  work  of 
Dulong  and  Petit  was  far  from  accu- 
rate, and  the  results  were  quite  prop- 
erly received  with  caution  and  conservatism  by  Berzelius 
and  others.  There  were  many  experimental  difficulties 
in  the  way  of  the  determinations.  A  high  degree  of  purity 
in  the  substance  tested  was  essential,  and  furthermore  as 
the  methods  became  more  accurate  it  was  seen  that  the 
specific  heat  of  a  substance  was  not  constant  for  all  changes 
of  conditions.  Hence,  in  I834,1  Avogadro  spoke  of  the 
law  as  an  approximation  only.  The  very  careful  work  of 
Regnault,  beginning  in  1840,  showed  in  how  far  this  was 
true.  It  is  known  now  that  the  specific  heat  increases 
with  increase  of  temperature  ;  that  for  the  same  substance 
it  is  greater  in  the  state  of  a  liquid  than  in  the  solid  state. 
In  the  case  of  metals,  it  is  diminished  by  rendering  them 
more  dense,  as  by  pressure,  and,  in  the  case  of  the  allo- 
tropic  forms  of  an  element,  the  specific  heat  is  often  dif- 
ferent even  under  similar  conditions.  These  matters  de- 
manded an  explanation  before  the  generalization  of  Dulong 
and  Petit  and  of  Neumann  could  be  accepted,  and  a  ben- 
eficial influence  be  exerted  upon  chemical  theory.  The 
painstaking  investigations  of  Regnault,  embracing  a  large 
number  of  substances,  confirmed  the  generalization  of 
Neumann  as  to  a  large  number  of  compounds,  and  proved 
that  the  law  applied  in  the  case  of  most  of  the  elements 

1  Ann.  chim.phys.,  55,  80  (1834). 


RELATIVE   WEIGHTS   OF  THE   ATOMS.  139 

examined — about  forty  in  all.     The  following  table  will 
illustrate  this : 


Element. 

Specific  heat. 

Atomic  weight. 

Sp.  ht.  X  at.  wt. 

Lithium  

0.941 

7 

6.6 

0.293 

23 

6.7 

0.250 

24 

6.0 

Potassium  

0.166 

39 

6.5 

Iron  
Copper  

0.114 
0.0952 

f 

63.1 

6.4 
6.0 

Zinc  

0.0955 

65 

6.2 

Silver  

0.0570 

108 

6.2 

Tin  

0.0562 

117.4 

6.6 

Gold  

0.0324 

197 

6.4 

Mercury  

0.0317 

200 

6-3 

0.0307 

207 

6.4 

207.3 

6.5 

From  many  determinations  it  is  seen  that  the  number 
obtained  by  multiplying  the  specific  heat  by  the  atomic 
weight  ranges  from  5  to  7,  averaging  about  6.25.  This 
then  represents  the  capacity  of  the  atom  for  heat  and  is 
called  the  atomic  heat.  The  variations  in  this  number, 
which  according  to  law  should  be  a  constant,  may  be  as- 
signed to  various  causes.  In  the  first  place,  the  atomic 
weights  are  far  from  being  accurately  known,  and  are  sub- 
ject always  to  errors  of  observation,  as  are  also  the  deter- 
minations of  the  specific  heats.  Again,  some  of  the  ele- 
ments have  never  been  obtained  in  a  condition  which  could 
be  with  certainty  regarded  as  pure.  Furthermore,  as  Reg- 
nault  observed,1  the  determination  is  uncertain,  as  it  in- 
cludes several  unknown  factors  which  so  far  can  not  be 
discriminated  between,  as  the  latent  heat  of  expansion 
and  of  fusion,  which  is  gradually  absorbed  by  bodies  as 
they  frequently  soften  long  before  the  temperature  is 
reached  which  is  regarded  as  their  melting-point.  Thus 
some  of  the  heat  goes  to  preparing  the  way  for  a  change 
of  aggregation  by  diminishing  the  force  of  cohesion. 
It  is  the  effect  of  the  sum  of  all  these  changes,  and  perhaps 

1  Ann.  chim.phys,,  3  ser.,  26,  262. 


140  A  STUDY   OF  THE   ATOMS. 

of  others  as  yet  unrecognized,  which  is  called  the  specific 
heat,  and  yet,  as  Wurtz  says,  it  is  surely  remarkable  that 
in  spite  of  the  complexity  of  the  phenomena  so  simple  and 
so  great  a  law  should  be  evolved  from  such  determinations.1 

It  will  render  this  part  of  the  subject 
Exceptions  somewhat  clearer  if  the  cases  be  examined 

in  which  wide  variations  from  the  average 
atomic  heat  have  been  observed.  The  elements  exhibit- 
ing these  variations  are  chiefly  carbon,  silicon  and  boron. 


Carbon  : 
(i    Diamond  ...... 

Specific 
heat. 

•    0.064 

Temperature, 
at      —50.5° 

Atomic 
weight. 

12 

Atomic 
heat. 

0-77 

0.096 

at 

—10.6° 

12 

I.I5 

0.113 

at 

—  10.7° 

12 

1.36 

0.132 

at 

-33-4° 

12 

1.58 

0.153 

at 

-58.4° 

12 

1.84 

0.177 

at 

-85-5° 

12 

2.12 

0.222 

at 

140.0° 

12 

2.66 

0.273 

at 

206.1° 

12 

3-28 

0-303 

at 

247.0° 

12 

3^4 

0.441 

at 

606.7° 

12 

5.29 

0.449 

at 

806.5° 

12 

5-39 

0-459 

at 

985-0° 

12 

5  5i 

b.  Graphite  

O.II4 

at 

50-3° 

12 

i-37 

0.199 

at 

61.3° 

12 

2-39 

0.297 

at 

201.6° 

12 

3.56 

0-445 

at 

641.9° 

12 

5-34 

0.467 

at 

977.9° 

12 

5-60 

c.  Charcoal  

0.194 

o 

to    99.2° 

12 

2-33 

0.239 

o  to  223.6° 

12 

2.87 

Silicon  : 

Crystallized  

0.136 

at 

-39-8° 

28 

3-81 

0.170 

at 

21.6° 

28 

4.76 

0.183 
0.190 

at 
at 

57-1° 
86.0° 

28 
28 

5-12 
5.32 

0.196 

at 

128.7° 

28 

5-49 

0.201 

at 

184.3° 

28 

5.63 

0.203 

at 

232.4° 

28 

5-68 

Boron  : 

Crvstallized 

0.192 

at 

-39-6° 

II 

2.  II 

0.238 

at 

26.6° 

II 

2.62 

0.274 

at 

76.7° 

II 

3.01 

0.307 
0.338 

at 
at 

125.8° 
177-2° 

II 
II 

3.38 
3-72 

0.366 

at 

233.2° 

II 

4-03 

1  "Atomic  Theory,"  p.  126. 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  141 

Manifestly,  at  ordinary  temperatures,  these  elements 
do  not  even  approximately  follow  the  law.  The  specific 
heats,  however,  increase  with  the  temperature,  and  at 
high  temperatures  they  are  in  accord  with  what  would 
be  required  by  the  law.  They  do  not  increase  beyond 
this  point.  The  specific  heats  vary  with  the  temperature, 
therefore,  and  for  each  element  there  seems  to  be  a  tem- 
perature beyond  which  variations  are  slight  and  at  which 
the  specific  heat  is  approximately  in  accord  with  the  law. 
Hence  the  generalization  can  be  assumed  to  be  a  law  only 
within  certain  fixed  limits  of  temperature.  From  all  of 
this  it  is  seen  that  the  law  of  Dulong  and  Petit  is  only  an 
approximation,  and  can  only  be  such  until  a  distinction 
can  be  made  between  the  heat  that  goes  to  increase  the 
temperature,  and  that  which  is  utilized  in  internal  work  in 
the  molecules.  In  the  case  of  solids  and  liquids  the  external 
work  is  probably  very  small.  On  the  other  hand,  the  inter- 
nal work  will  vary  with  the  size  of  the  molecule  and  may 
amount  to  a  good  deal.  The  fact  that  it  does  not  cause  a 
greater  divergence  from  the  law  on  the  part  of  many  ele- 
ments would  seem  to  indicate  that  it  also,  like  the  specific 
heats,  is  inversely  proportional  to  the  atomic  weights. 

In  making  use  of  the  law  of  Dulong  and  Petit  for  de- 
ducing atomic  weights,  it  is  necessary  to  prove  that  the 
specific  heat  has  been  determined  at  temperatures  between 
which  it  shows  but  slight  variations,  and  the  range  of 
temperature  should  be  a  wide  one. 

In  conclusion,  certain  further  variations 

F*al!!ir?S  may  be  mentioned.     The  law  does  not 

of  the  Law.         ,    . ^         ,  ,  ,.,     , 

hold  good  for  gaseous  elements,  like  hy- 
drogen and  oxygen,  nor  for  gaseous  compounds.  In  the 
case  of  several  elements,  the  specific  heats  in  the  solid 
state  are  about  twice  as  great  as  in  the  gaseous  state. 


142  A   STUDY   OF  THE   ATOMS. 

These  facts  but  emphasize  the  statement  already  made 
that  the  so-called  law  of  Dulong  and  Petit  is  in  the  truest 
sense  only  an  approximation,  and  so  long  as  the  deter- 
minations yield  results  which  are  the  sum  of  several 
unknown  quantities,  it  is  unscientific  and  untrue  to  call 
the  generalization  a  law.  It  is  far  from  proved  that  the 
atoms  of  the  various  elements  have  the  same  capacity  for 
heat.  It  can  only  be  maintained  that  under  certain  fixed 
conditions,  and  judged  by  our  systems  of  measurement,  the 
variations  from  a  certain  constant  are  not  great,  and  con- 
sequently this  constant  may  be  used  in  deciding  the  num- 
ber of  atoms  in  a  molecule  and  in  coming  to  a  decision 
between  two  or  more  possible  atomic  weights. 

It  is  of  importance  here  to  note  the  hy- 
Hypothesis  pOthesis  suggested  by  Kopp.  It  was 

offered  in  explanation  of  the  wide  devia- 
tions shown  by  certain  elements  as  carbon,  silicon  and 
boron  before  the  influence  of  temperature  upon  them  was 
known,  and  so  to  make  it  consistent  with  later  knowl- 
edge. Lothar  Meyer1  has  modified  it  somewhat.  Let  it 
be  supposed  that  elementary  atoms  are  composed  of  still 
smaller  parts,  which  may  be  called  particles,  and  that  at 
low  temperatures  the  motion  of  these  particles  is  that  of  a 
single  system.  At  higher  temperatures  this  system  is 
resolved  into  others  containing  a  smaller  number  of  par- 
ticles. And  finally,  at  still  higher  temperatures  this 
resolution  takes  place  to  such  an  extent  that  each  par- 
ticle moves  freely  and  independently.  Therefore  at 
temperatures  at  which  the  atoms  do  not  obey  the  law  of 
Dulong  and  Petit  the  particles  do  not  move  singly  but  in 
groups  of  several  such  particles,  each  of  which  requires 
the  same  amount  of  heat  to  raise  its  temperature  through 

1  "Modern  Theories,"  p.  93. 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  143 

i°  as  is  required  by  the  single  particle  of  an  atom  sub- 
servient to  this  law.  For  example,  an  atom  of  carbon  in 
the  form  of  diamond,  possessing  at  — 50°  C.  an  atomic 
heat  of  0.76,  contains  half  the  number  of  groups  of  par- 
ticles which  it  contains  at  27.7°,  at  which  temperature  its 
atomic  heat  is  twice  as  large,  viz. ,  1.52.  The  different 
groups  are  not  of  necessity  twice  as  large  but  must,  on  an 
average,  contain  twice  as  many  particles  at  — 50°  C.  as 
they  contain  at  27.7°  C. 

Tilden1  has  made  certain  specific  heat  determinations 
for  nickel  and  cobalt  at  very  low  temperatures  ( — 78.4° 
and  — 182.4°)  which  led  him  to  think  that  at  absolute 
zero  the  products  of  the  specific  heats  multiplied  by  the 
atomic  weights  would  be  identical  or  differ  only  by  the 
very  small  amounts  due  to  experimental  error.  Further 
experiments  with  silver,  copper,  iron  and  aluminum 
failed,  however,  to  justify  this  expectation. 

Another  generalization  announced  in 
Law  of  Iso-  ig  used  for  the  Determination  of  the 
morphism.  =7  . 

number  of  atoms  in  solid  molecules,  was 

that  called  the  L,aw  of  Isomorphism  and  discovered  by 
Mitscherlich.  This  also  has  proved  to  be  no  law  in  the 
truest  sense,  but  an  approximation  and  one  of  far  more 
limited  application  than  that  of  Dulong  and  Petit.  Yet 
at  first  it  was  hailed  with  acclaim  and  was  considered  one 
of  the  most  important  aids  toward  determining  the  num- 
ber of  atoms  in  the  molecule. 

The  work  of  Mitscherlich  had  for  its  basis  many 
observations  of  a  long  line  of  chemists,  going  back  even 
to  the  time  of  Stahl,  for  chemists  having  few  reliable 
criteria  at  command  had  long  observed  the  crystal  forms, 
especially  of  minerals,  most  closely  as  affording  an  indi- 

<     i  Bakerian  Lecture  before  Royal   Society,  March  8,   1900 ;   Chetn.  News* 
81-133- 


144  A   STUDY  OF  THE   ATOMS. 

cation  of  similarity  or  unlikeness  of  composition.  Bodies 
which  crystallized  in  different  forms  even  though  other- 
wise alike  were  often  believed  to  differ  in  composition. 
And  yet  against  this  conclusion  were  known  such  facts 
as  the  qualitative  and  quantitative  identity  of  arragonite 
and  calcite  which  differ  in  crystal  form  ;  and  of  anatase 
and  rutile.  Hauy  had  maintained  that  the  crystalline 
figure  was  dependent  upon  the  form  of  the  smallest  par- 
ticle and  that  these  must  have  a  constant  composition. 
This  was  disputed  by  Berthollet  and  led  to  a  prolonged 
discussion. 

Several  theories  were  advanced  by  earlier 
y?r  y .  chemists  to  explain  the  fact  that  two  or 

more  bodies  may  have  the  same  crystal 
form  though  differing  in  composition.  Probably  the  most 
noteworthy  one  at  the  time  that  Mitscherlich  announced 
his  theory  was  that  the  form  was  assumed  through  the 
influence  of  some  impurity.  Thus  the  natural  occurring 
carbonates  of  magnesium,  zinc,  iron,  etc.,  crystallize  in 
the  same  form  as  calc-spar  and  it  was  believed  that  this 
was  due  to  the  presence  of  small  amounts  of  calcium 
carbonate  in  these  bodies,  but  when  calcium  carbonate 
itself  crystallized  as  arragonite  this  was  supposed  to  be 
due  to  its  containing  some  strontium  carbonate,  this  hav- 
ing a  superior  determining  force  to  calcium  carbonate. 

When  it  was  shown  that  strontium  carbonate  was  not 
to  be  detected  in  many  specimens  of  arragonite,  Gay- 
L,ussac  drew  attention  to  the  growth  of  one  substance 
upon  a  crystal  of  another  as  a  particularly  important  phe- 
nomenon in  considering  this  question.  Thus  a  crystal  of 
potassium  alum  placed  in  a  solution  of  ammonium  alum 
would  continue  to  grow  without  change  of  form.  This 
he  said  must  be  due  to  the  fact  that  the  two  alums  have 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  145 

particles  of  the  same  form  and  are  endowed  with  the  same 
energies.  In  1817  Beudant  returned  to  the  theory  of  the 
form  of  the  crystal  being  determined  by  some  mixture  or 
impurity,  basing  his  views  especially  on  the  behavior  of 
the  salts  then  called  vitriols.  Thus,  in  a  mixture  of 
copper  and  iron  sulphates,  the  crystals  take  the  form  of 
the  latter,  even  though  it  may  form  but  9  per  cent,  of 
the  mixture.  In  mixtures  of  zinc  and  iron  sulphates, 
the  latter  determines  the  form  when  present  to  the  amount 
of  15  per  cent.  In  mixtures  of  the  three,  as  little  as  3 
per  cent,  of  iron  sulphate  is  sufficient  to  determine  the 
form.  In  these  experiments  no  account  was  taken  of  the 
water  of  crystallization. 

These  theories  were  substituted  in  1819 

mteterUch.        by   the  theory   of   Mftsd"*11011-1    He 
had  been  busied  with  an  investigation 

of  the  phosphates  and  arsenates.  At  the  beginning  of 
his  report  upon  this  work  to  the  Berlin  Academy,  he 
wrote  that  it  seemed  to  him  certain  that  the  agreement  in 
chemical  behavior  which  the  compounds  show  that  are 
constituted  in  equal  proportions  and  with  like  crystal  form 
may  scarcely  be  referred  to  the  agreement  in  crystalliza- 
tion as  its  ground ;  that  they  lead  us  rather  to  a  more 
deeply  hidden  cause  by  which  both  the  composition  of 
the  body  and  the  agreeing  crystallization  are  to  be  ex- 
plained. In  the  case  of  phosphate  and  arsenate  of  the 
same  base  he  found  the  crystal  form  to  be  identical.  This 
he  at  first  attributed  to  their  containing  the  same  number 
of  atoms.  He  investigated  the  sulphates  and  found  that 
where  they  crystallized  differently  they  had  different 
amounts  of  water  of  crystallization.  When  they  crystal- 
lized together  or  in  mixtures  they  always  contained  the 

i  Abhandlungen  d.  Berl.  Akad.,  1819,  p.  426  ;  Ann.  chim.  phys.,  14,  172. 


146  A  STUDY  OF  THB  ATOMS. 

same  amount  and  so  assumed  the  form  of  the  sulphate 
which  corresponded  to  this.  Further  study  convinced 
Mitscherlich  that  the  number  of  the  atoms  was  not  the 
only  thing  to  be  considered  but  that  their  nature  was  a 
controlling  factor.  He  recognized  that  there  were  certain 
elements,  called  by  him  isomorphous,  which  gave  com- 
pounds of  identical  crystal  form  by  uniting  with  the  same 
number  of  atoms  of  other  elements.  These  he  placed  in 
groups.  This  is  to  be  noticed  as  one  of  the  earliest 
recognitions  of  families  of  elements.  This  identity  of 
crystal  form  was  dependent  upon  a  similarity  in  the 
arrangement  of  the  atoms.  If  the  conditions  of  crystal- 
lization were  changed,  different  forms  might  be  obtained. 
This  would  account  for  the  dissimilarity  of  form  in  the 
case  of  arragonite  and  calc-spar.  This  he  styled  poly- 
morphism. In  1821  he  formulated  his  hypothesis  as  fol- 
lows i1 

An  equal  number  of  atoms  combined  in  the  same 
manner,  gives  the  same  crystalline  form  ;  this  crystalline 
form  is  independent  of  the  chemical  nature  of  the  atoms, 
depending  only  on  their  number  and  arrangement. 

Of  course  the  chemical  nature  of  the  elements  deter- 
mines the  number  and  arrangement  of  the  atoms  in  the 
molecule  and  so  influences  the  crystalline  form,  but 
Mitscherlich  believed  it  to  be  without  direct  influence. 

It  is  clear  that  if  this  hypothesis  is  true  it  affords  a 
most  valuable  method  for  determining  the  number  of  atoms 
in  the  molecule  and  so  of  deciding  upon  an  atomic  weight 
which  may  be  in  doubt.  Given  a  compound  containing 
a  known  number  of  atoms  with  known  atomic  weights, 
and  the  number  of  atoms  in  any  compound  crystallizing 
in  the  same  form  could  be  determined.  Berzelius  made 
use  of  the  law  of  isomorphism  in  deciding  the  atomic 

1  Ann.  chim.  ph-ys.,  19,  419. 


RELATIVE  WEIGHTS   OF  THE  ATOMS.  147 

weights  for  his  tables  and  placed  more  confidence  in  his 
results  obtained  by  this  method  than  by  any  others.  But 
he  found  that  in  this  also  there  was  much  uncertainty 
and  was  forced  to  alter  in  a  number  of  cases  the  figures 
selected  as  the  atomic  weights. 

There  were  several  reasons  for 

UncertlhT  DraW"        this  ™<*rtainty  in  conclusions 

drawn  from  the  law  of  isomor- 

phism. Many  instances  may  be  adduced  in  which  the 
compounds  showing  identity  of  crystal  form  undoubtedly 
contain  different  numbers  of  atoms.  Again  similarity  in 
number  and  arrangement  of  atoms  does  not  always  pro- 
duce identity  of  form.  In  the  effort  at  eliminating  such 
cases,  which  do  not  follow  the  generalization,  the  definition 
of  isomorphism  has  been  changed  and  limited.  Thus  it 
was  stated  that  mere  identity  of  form  was  insufficient  to 
prove  isomorphism.  It  should  further  be  required  that 
the  substances  should  crystallize  together  and  in  varying 
proportions  be  able  to  build  up  one  and  the  same  crystal; 
that  is,  that  a  crystal  of  one  substance  should  continue  to 
grow  in  a  solution  of  the  other.  This  possibility  of  over- 
growth has  been  accepted  by  many  as  the  best  proof  of 
isomorphism  but  this  involves  immediately  an  anomaly 
since  this  overgrowth  is  especially  noted  in  compounds  of 
potassium  and  ammonium  where  equality  of  atomic  com- 
position is  impossible. 

There  are  many  facts  which  render  the 
correlation  of  Crystal  form  and  chemi- 


cal composition  a  very  complex  prob- 
lem. There  has  been  some  attempt  at  differentiating  be- 
tween the  facts  and  classifying  the  data.  Thus  there  are 
what  have  been  called  the  phenomena  of  "homeomor- 
phism"  where  there  is  a  difference  in  composition  but  an  ap- 


148  A   STUDY   OF   THE   ATOMS. 

proximation  as  to  form.  Many  instances  of  homeomor- 
phism  might  be  given  ;  thus  arragonite,  CaCO8,  and  nitre, 
KNO3;  baryte,  BaSO4,  potassium  permanganate,  KMnO4, 
and  potassium  perchlorate,  KC1O4.  Again,  as  Dana  has 
pointed  out,  there  are  instances  with  still  greater  dissimi- 
larity of  composition  ;  thus,  cinnabar,  HgS,  and  susan- 
nite,  PbSO4,3PbCO3  ;  potassium  hydrogen  sulphate, 
KHSO4,  and  feldspar,  KAlSi3O8;  etc. 

It  is  manifest  that  mere  nearness  of  crystalline  form 
will  not  answer.  It  is  probable  that  the  limitation  to  the 
capacity  for  overgrowth  does  not  bring  any  nearer  the 
solution  of  the  question  as  to  the  influence  of  atomic  com- 
position upon  form.  It  would  seem  that  a  solution  is  to 
be  reached  only  by  most  accurate  determinations  of  angles 
of  crystals,  and  the  changes  produced  in  these  by  varying 
the  atomic  composition.  Work  along  this  line  has  been 
done,  and  it  is  already  evident  that  with  a  definite  change 
of  composition  certain  angles  remain  constant  though 
others  may  be  altered  greatly.  Further  work  along  this 
line  is  very  necessary  and  would  seem  to  promise  impor- 
tant results.  The  hypothesis  of  Mitscherlich  is  clearly  not 
to  be  called  a  law,  but  is  a  generalization  of  somewhat 
uncertain  and  limited  application,  and  while  it  has  been  a 
valuable  aid  in  the  determination  of  atomic  weights,  chiefly, 
as  Wurtz1  says,  when  its  indications  can  be  connected  with 
positive  intelligence  drawn  from  the  law  of  volumes  or 
the  law  of  specific  heats,  greater  interest  now  attaches  to 
the  wider  question  as  to  the  correlation  in  general  between 
crystal  form  and  atomic  composition. 

Electricity  had  been  used  since  the 

last  Part  Of  the  I8th  century  as  a 
powerful  agent  for  bringing  about 

the  decomposition  of  chemical  substances.     Thus  the  de- 

1  Wurtz  :  "Atomic  Theory,"  p.  148. 


RELATIVE   WEIGHTS   OF   THE   ATOMS.  149 

composition  of  water  was  studied  by  a  number  of  obser- 
vers, and  a  little  later  there  followed  the  brilliant  decom- 
position of  the  alkalies  by  Davy  with  the  production  of 
the  alkali  metals.  In  1803  Berzelius  and  Hisinger  showed 
that  the  passage  of  a  current  of  electricity  through  a  salt 
separated  the  acid  from  the  base,  the  former  being  found 
at  the  positive  pole  and  the  latter  at  the  negative.  Davy's 
work  confirmed  this,  but  this  work  was  almost  exclusively 
qualitative  until  Faraday  studied  the  changes  quantita- 
tively and  detected  the  connection  which  existed  with  the 
combining  numbers  of  the  elements,  thus  deducing  his 
law  of  electrical  equivalents. 

In  1834  Faraday1  showed  that  the  electrochemical  de- 
composition is  a  fixed  quantity  for  a  definite  amount  of 
electricity.  Out  of  various  compounds  subjected  to  this 
decomposition,  as  water-dissolved  hydracids,  fused  metal- 
lic chlorides,  etc. ,  equal  amounts  of  the  same  element  were 
separated  by  the  expenditure  of  the  same  amount  of  elec- 
tricity. The  amounts  of  different  elements  thus  separated 
correspond  with  their  ordinary  chemical  equivalents  or 
combining  numbers.  Hence  he  called  these  numbers  the 
electrochemical  equivalents.  They  may  be  further  re- 
garded as  the  relative  weights  of  the  atoms.  It  is  easy  to 
see  how  this  method  of  determination  might  be  used  in 
connection  with  the  methods  already  mentioned  to  confirm 
their  results.  But  after  all,  this  is  only  another  method  of 
analysis  and  without  a  knowledge  of  the  number  of  atoms 
in  the  molecule  of  the  compound,  the  solution  of  that 
problem  is  as  far  off  as  ever.  Faraday  pointed  out  that 
the  electrochemical  equivalents  obtained  for  bodies  which 
were  capable  of  direct  electrolysis  did  not  agree  in  many 
cases  with  those  assumed  by  Berzelius  nor  with  those  ob- 

1  Phil.  Trans.,  1834,  p.  77. 


150  A   STUDY   OF   THE   ATOMS. 

tained  by  the  use  of  the  specific  heats,  etc.  For  instance, 
the  electrochemical  equivalents  of  oxygen  and  chlorine 
did  not  stand  in  the  same  ratio  as  the  weights  of  equal 
volumes  of  these  two  elements. 

Several  methods  have  been  devised 

Fr*ezin?=Points  in  m<>re  recent  years  for  determining 
of  Solutions.  .  * 

the  number  of  atoms  in  molecules  of 

solids  dissolved  in  liquids.  When  a  solid  is  dissolved  in 
a  liquid,  the  freezing-point  of  the  solvent  is  lowered. 
The  experiments  of  Raoult  have  shown  that  this  bears  a 
definite  relation  to  the  molecular  weight  of  the  dissolved 
substance.  The  law  deduced  by  Raoult  was  that  for 
every  molecule  of  a  compound  dissolved  in  100  molecules 
of  a  liquid,  the  freezing-point  is  lowered  by  an  approxi- 
mately constant  amount,  namely,  0.62°.  If  P  =  weight 
of  compound,  I/  =  weight  of  solvent,  E  =  lowering  of 
freezing-point,  m  =  molecular  weight  of  compound,  then 

P  X  M  .  0  P  X  62M 

K  — 0.62°,  orm  = 


I<  X  iooM  •"  L  X  E  ' 

This  so-called  law  does  not  hold  good  for  some  classes  of 
substances,  as  inorganic  salts,  strong  bases  and  acids. 
It  is  chiefly  used  with  organic  substances  and  organic  sol- 
vents. The  two  most  commonly  used  solvents  are  benzol 
and  acetic  acid. 

It  is  easy  to  see  that  a  similar 
generalization  could  probably 
be  drawn  as  to  the  vapor-pres- 
sures and  boiling-points  of  solutions.  The  solution  of  a 
substance  lowers  the  vapor-pressure.  This  also  was  ex- 
amined by  Raoult  and  others  and  the  following  generaliza- 
tion deduced.  The  relative  lowering  of  the  vapor- 
pressure  is  proportional  to  the  ratio  of  the  number  of 
molecules  in  solution. 


• 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  151 

In  the  case  of  the  boiling-point,  we  find  it  raised  and 
the  elevation  of  the  boiling-point  is  proportional  to  the 
concentration.  Where  we  have  equally  concentrated 
solutions  of  different  substances,  the  increase  in  the  boil- 
ing-point is  inversely  proportional  to  the  molecular 
weights  of  the  substances.  Lastly  it  has  been  shown  by 
Pfeffer  and  van't  Hoff  that  interesting  relations  obtain 
between  the  osmotic  pressure  and  the  molecular  weights 
of  dissolved  substances. 

The  lack  of  uniformity  in  the  atomic 
weiShts  and  the  general  uncertainty 
surrounding  them,  which  prevailed 
during  the  third  and  fourth  decades  of  the  igth  century, 
weakened  greatly  the  confidence  placed  in  the  atomic 
theory  so  that  many  were  ready  to  abandon  it,  and,  es- 
chewing theory,  devote  themselves  solely  to  the  practical 
side  of  the  science.  By  such,  the  name  combining  weight 
was  preferred  to  that  of  atomic  weight.  The  generaliza- 
tions of  Dulong  and  Petit  and  of  Mitscherlich  were  beset 
with  difficulties  and  their  exceptions  were  unexplained. 
The  theory  of  Avogadro  failed  to  clear  up  matters  so  long 
as  no  distinction  was  made  between  atoms  and  molecules. 
Each  method  seemed  to  yield  results  at  variance  with  the 
others.  Regnault,  using  the  specific  heats,  called  his 
'  'equivalents  thermiques' '  ;  Rose  and  Marignac  gave  tables 
of  "isomorphic  equivalents",  and  there  were  the  "electro- 
chemical equivalents' '  of  Faraday.  The  Berzelian  table  had 
oxygen  equal  to  100  for  its  standard  ;  the  Gmelin  table 
had  hydrogen  equal  to  i .  It  is  not  strange  that  confusion 
reigned. 

In  1837  Dumas1  drew  attention  to  the  dis- 

om  an  tinction   between   atoms    and    molecules. 

Yet  he  thought  the  idea  of  atomic  weight 

l  "  Philosophy  of  Chemistry,"  1837. 


152  A  STUDY   OP   THE   ATOMS. 

an  indeterminate  one  and  that  no  confidence  was  to  be 
placed  in  it.  The  equivalents  or  combining  numbers 
could  be  determined  by  analysis.  If  it  were  possible  he 
would  forever  banish  the  word  atom  from  chemistry  since 
he  was  persuaded  that  it  went  beyond  that  which  could 
be  fixed  by  experiment.  Liebig1  in  1839  expressed  him- 
self in  a  similar  manner.  The  equivalents,  he  said,  would 
never  change  but  he  very  much  doubted  whether  chem- 
ists would  ever  be  agreed  as  to  the  numbers  by  which  the 
relative  atomic  weights  should  be  expressed.  The  study 
of  chemistry  would  be  made  greatly  easier  when  all 
chemists  decided  to  return  to  the  use  of  equivalents. 

It  is  not  clear  just  how  the  return  to  the  equivalents, 
contended  for  years  before  by  Wollaston,  was  to  do  away 
with  the  confusion  and  lack  of  uniformity.  It  is  evident 
that  there  would  have  to  be  a  choice  made  between 
possible  equivalents  just  as  it  must  be  made  between 
possible  atomic  weights  and  there  were  no  better  guides 
to  a  choice  in  the  one  case  than  in  the  other.  The  neglect 
of  the  practical  distinction  between  atoms  and  molecules 
continued  for  two  more  decades.  Thus  there  is  no  ap- 
parent distinction  made  by  Graham  and  others  of  his 
time,  but  those  who  were  especially  busied  with  organic 
chemistry  were  beginning  to  see  more  clearly.  The  uni- 
tary system  of  Gerhardt  was  coming  in,  displacing  the 
dualism  of  Berzelius. 

The  systems  of  numbers  in  use  gradually  narrowed 
down  to  two,  though  variations  as  to  special  elements 
were  not  infrequent.  These  two  chief  systems  were  those 
of  Berzelius  and  of  Gerhardt.  One  of  the  foundations  of 
the  Berzelian  system  was  the  law  of  volumes  as  erroneously 
interpreted  by  him.  This  erroneous  interpretation  was 

1  Ann.  d.  Phartn.,  31,  36. 


RELATIVE   WEIGHTS   OF  THE   ATOMS.  153 

practically  overthrown  by  the  work  of  Dumas  and  Mit- 
scherlich  upon  vapor-densities.  Mitscherlich's  discovery 
of  isomorphism  caused  Berzelius  to  make  important 
changes  in  his  earlier  tables.  He  introduced  the  term 
'  double  atoms'  to  allow  for  the  exceptions  to  his  idea  of 
the  law  of  volumes  and  to  make  certain  that  his  atomic 
weights  correspond  with  the  numbers  more  generally  ac- 
cepted by  the  leading  chemists.  Thus  hydrogen,  nitro- 
gen, chlorine,  bromine  and  iodine  were  classed  among 
the  double  atoms  and  were  supposed  to  enter  into  combi- 
nation in  pairs,  each  pair  representing  what  other  chemists 
styled  an  equivalent.  This  was  very  awkward  and  did 
not  appeal  to  the  better  judgment  of  most  chemists. 
Dal  ton  and  Thomson,  Gay-Lussac,  Wollaston  and  lastly 
Gmelin,  whose  "  Handbook"  exercised  a  wide-spread  in- 
fluence, adopted  the  numbers  obtained  from  equivalent 
quantities  which  enter  into  combination.  The  law  of 
volumes  was  entirely  discredited.  Chemical  analysis  was 
largely  relied  upon.  The  widespread  popularity  of 
Gmelin' s  "  Handbook"  secured  a  large  following  for  his 
system  of  weights.  In  the  earlier  editions  of  the  Hand- 
book, these  were  called  by  the  unfortunate  name  of 
mixing  weights  ( Mischungsgewichte) .  The  name  was 
later  changed  to  atomic  weights  without  making  any 
material  change  in  the  notation.  The  chief  point  of  dis- 
cussion between  Gmelin  and  Berzelius  lay  in  the  correct- 
ness of  the  weights  which  were  halved  by  Berzelius  and 
the  propriety  of  the  assumption  of  double  atoms,  these 
double  atoms  being  the  true  atoms  in  the  opinion  of 
Gmelin.  The  law  of  volumes  was,  he  maintained,  con- 
tradicted by  experiment  and  therefore  not  a  reliable  guide 
in  this  matter.  Again  the  half  atoms  never  entered  into 
combination  and  consequently  their  assumption  was  un- 


154  A  STUDY  OF  THE  ATOMS. 

necessary.  Berzelius  would  make  the  formula  for  water 
H2O,  hydrochloric  acid  H2C12,  and  ammonia  H6N2. 
Gmelin  would  double  the  atomic  weight  of  hydrogen  and 
so  simplify  these  formulas  to  HO,  HC1,  H3N. 

Gmelin  ably  defended  his  system  in  1843  and  it  was 
adopted  by  L,iebig  and  by  almost  all  chemists.  With  the 
development  of  organic  chemistry,  however,  it  began  to 
be  apparent  that  there  was  to  be  a  return  to  the  law  of 
volumes  which  had  been  completely  sacrificed  in  the  tables 
of  Gmelin.  This  was  first  seen  by  Gerhardt  and  he 
brought  most  influential  support  to  the  system  of  Berze- 
lius, at  the  same  time  introducing  much  needed  correc- 
tions. His  first  and  most  important  follower  was  Laurent. 

In  1858,  Cannizzaro  proposed  the  doub- 

ling  °f  many  °f  the  atomic  weiShts  to 
bring  them  into  harmony  with  the  the- 
ory of  Avogadro  and  the  law  of  specific  heats.  He  in- 
sisted on  a  clear  distinction  being  maintained  between 
atoms  and  molecules  and,  with  this  distinction  kept  in 
yiew,  the  difficulties  in  the  way  of  the  acceptance  of  the 
theory  of  Avogadro  disappeared.  His  views  were  based 
upon  the  work  and  conceptions  of  Avogadro,  Regnault 
and  Gerhardt.  The  theory  of  Avogadro,  if  true,  must 
be  the  surest  means  of  deciding  upon  the  atomic  weights- 
These  views  of  Cannizzaro  were  given  in  his  course  of 
lectures  and  in  the  form  of  a  letter  to  his  colleague,  Luca, 
professor  of  chemistry  at  Pisa. 

In  1860,  a  congress  of  chemists  was 
Congress  of  called  at  Carlsruhe  to  put  an  end  to  the 
Carlsruhe.  ...  ,,.  ,  ,.  ,  •  <.  j  u 

confusion  and  discord  which  existed  be- 
tween the  diverse  systems.  Dumas  presided.  He 
accepted  the  atomic  weights  of  Berzelius  with  modifica- 


RELATIVE   WEIGHTS   OF  THE   ATOMS.  155 

tions  indicated  by  Regnault  and  Rose  and  Marignac,  but 
opposed  the  introduction  of  the  hypothesis  of  Avogadro. 
And  yet  in  1826  Dumas  had  published  an  important 
memoir,  (<Sur  quelques  points  de  la  theorie  atomistique" 
in  which  he  had  taken  as  his  starting-point  the  theory  of 
Avogadro  in  his  search  for  a  means  of  harmonizing  the 
deductions  from  specific  heats  and  isomorphism.  This 
work  had  so  impressed  Cannizzaro  that  he  had  called  it 
the  theory  of  Avogadro,  Ampere  and  Dumas.  Hence 
his  surprise  was  great  at  this  opposition  of  Dumas,  but 
so  strong  was  the  prestige  and  influence  of  Dumas  that 
the  congress  reached  no  agreement.  As  Cannizzaro  says,1 
the  delegates  separated  without  having  passed  any  reso- 
lution and  each  one  persisting  in  his  opinion.  Attention 
had  been  called,  however,  to  the  work  of  Cannizzaro  and 
in  a  few  years  his  conclusions  were  accepted  by  the 
majority  of  chemists.  The  change  thus  introduced  in 
the  atomic  weights  made  the  discovery  of  the  Periodic 
Law  possible. 

Since  then  the  theoretical  principles  observed 
pfl  er  in  atomic  weight  determinations  have  re- 

mained the  same.  The  theory  of  Avogadro 
has  approved  itself  as  of  the  greatest  value,  the  specific 
heats  have  been  repeatedly  appealed  to,  and  less  often  the 
testimony  of  isomorphism.  Improved  analytical  methods 
have  introduced  the  greater  number  of  changes,  though 
much  is  still  needed  along  this  line.  A  fuller  knowledge 
of  the  chemical  behavior  and  analogies  of  the  elements 
has  brought  about  a  number  of  corrections,  the  greatest 
help  along  this  line  coming  through  the  discovery  of  the 
Periodic  Law,  an  account  of  which  is  to  follow  in  the 
next  chapter. 

1  "Les  Actualites  chimiques,"  II,  12. 


156  A   STUDY   OF  THE   ATOMS. 

Each  of  the  later  decades  of  the  igth  century  has  seen 
steady  improvement  until,  from  a  condition  of  great  con- 
fusion and  wide  variation,  a  fair  uniformity  has  been 
attained  in  the  atomic  weights  accepted  by  chemists  of 
all  nationalities.  The  differences  are  now  mainly  due  to 
differences  in  judgment  as  to  the  relative  value  of  experi- 
mental determinations  coming  from  various  sources. 
Entire  unanimity  can  only  be  attained  by  agreement  be- 
tween representatives  of  the  great  national  societies  of 
chemists.  Such  unanimity,  however,  must  not  be  mis- 
taken for  actual  approximation  to  the  truth.  Unquestion- 
ably a  considerable  number  of  the  atomic  weights  still 
rest  upon  very  slim  evidence  and  much  careful  work  is 
still  needed. 

The  so-called  atomic  weights  are, 
Standard  for  the  Q£  CQ  not  absolute  but  reiative. 
Atomic  Weights. 

They  are  after  all  to  be  considered 

as  combining  weights,  and  for  purposes  of  comparison  a 
standard  is  necessary.  The  discussion  as  to  the  best 
available  standard  has  been  going  on  for  nearly  a  century. 
This  has  little  bearing  upon  theory  and  is  chiefly  inter- 
esting because  of  its  practical  application  in  calculations. 
It  is  not  necessary  then  to  go  into  an  extended  historical 
account  of  the  discussion.  A  brief  resum£  will  be  suffi- 
cient. 

For  the  first  table  of  atomic  weights  as 
L®  d  d  given  by  Dalton,  hydrogen  was  taken  as 

the  standard  and  i  was  the  value  assigned 
to  it.  This  choice  was  doubtless  determined  by  the  be- 
lief that  the  hydrogen  atom  was  the  lightest  and  there- 
fore all  the  other  atomic  weights  would  be  represented 
by  figures  greater  than  unity. 

The  choice  was  confirmed  by  the  hypothesis  which 


RELATIVE   WEIGHTS   OF  THE   ATOMS.  157 

soon  arose  that  hydrogen  was  possibly  the  component  of 
the  other  atoms.  This  was  seen  in  Dalton's  own  work 
and  in  the  hypothesis  of  Prout,  which  received  wide 
credence  and  which  has  stubbornly  resisted  dislodgment 
from  the  minds  of  influential  chemists.  Hydrogen  is 
therefore  to  be  known  as  the  Dal  ton  standard. 

The  clear  vision  of  Berzelius,  to  whom 
Stancf'rd  chemists  are  so  largely  indebted  for  the 

sure  and  safe  foundation  of  their  science, 
soon  saw  that  more  important  reasons  were  to  be  con- 
sidered than  mere  convenience  or  a  sentimental  regard 
for  a  unit  standard.  Above  all  things  accuracy  was  de- 
manded in  these  constants  on  which  chemical  work  and 
chemical  theory  were  to  rest.  The  atomic  weights  repre- 
sented ratios  to  the  standard.  All  error  could  not  be 
avoided  in  the  experimental  determinations  of  these 
ratios,  but  the  error  would  be  simple  and  not  duplicated 
where  the  ratio  was  directly  determined.  Therefore  that 
element  should  be  taken  as  the  standard  which  gave 
direct  ratios  with  the  largest  number  of  elements.  Under 
this  provision  but  one  element  could  be  chosen,  namely 
oxygen.  Should  any  other  element,  as  hydrogen,  be 
chosen,  then  the  ratio  of  this  element  to  oxygen,  as  well 
as  the  ratio  of  oxygen  to  the  element  in  question,  would 
have  to  be  determined  and  thus  the  error  in  the  latter 
multiplied  by  the  error  in  the  former  determination. 
However  often  the  oxygen-hydrogen  ratio  may  be  re- 
vised, error  is  inevitable.  It  is  a  simple  ordinary  pre- 
caution against  error,  therefore,  to  discard  the  unneces- 
sary use  of  the  oxygen-hydrogen  ratio  and  to  take  the 
direct  ratio  as  final.  It  may  be  added  that  hydrogen 
would  be  an  especially  poor  choice  for  the  direct  ratios  as 
less  than  half  a  dozen  such  have  been  satisfactorily  de- 


158  A  STUDY  OF  THE   ATOMS. 

termined.  If  the  error  in  the  oxygen-hydrogen  ratio  be 
supposed  to  be  only  o. i  per  cent.,  and  this  is  really  less 
than  the  probability,  then  we  would  have  an  error  of  over 
i  per  cent,  in  many  of  the  higher  atomic  weights  apart 
from  the  error  due  to  experiment. 

The  necessity  then  for  choosing  oxygen  as  the  stand- 
ard could  not  fail  to  impress  itself  upon  so  clear-sighted 
a  chemist  as  Berzelius,  and  it  would  be  a  further  recom- 
mendation to  his  mind  that  the  oxygen  standard  would 
be  a  protest  against  the  wild  hypothesis  of  Prout.  The 
hydrogen  standard  was,  however,  preferred  by  many  chem- 
ists and  was  the  only  one  in  common  use  during  the 
greater  part  of  the  i9th  century. 

Due  consideration  of  the  points 

lhe«Xall!f  A/signed  mentioned  above  have  convinced 
the  Standard.  .  .  . 

the  majority  or  chemists  of  the 

present  day  that  the  proper  standard  is  oxygen.  It  only 
remains  to  assign  it  a  value.  Some  have  contended  that 
a  standard  must  be  the  unit  also.  This  is  a  custom  which 
has  been  departed  from  in  many  cases  and  need  not  be 
binding  upon  chemists  if  it  involves  inconveniences  and 
inaccuracies.  The  adoption  of  the  value  i  or  10  for  oxy- 
gen would  involve  the  use  of  fractional  atomic  weights 
for  some  elements.  Wollaston  used  the  value  10  for 
oxygen  but  had  little  following.  Berzelius  used  the  value 
100  for  oxygen.  This  gives  us  a  large  number  of  atomic 
weights  of  inconvenient  size.  Roughly  speaking,  all  of 
the  atomic  weights  at  present  in  use  would  have  to  be 
multiplied  by  6  ^.  A  number  of  them  therefore  would 
lie  between  1200  and  1500.  This  probably  accounts  for 
Berzelius'  lack  of  success  in  establishing  oxygen  as  the 
standard.  Seeing  that  the  hydrogen  unit  predominated 
during  most  of  the  igth  century,  and  that  for  the  greater 


RELATIVE  WEIGHTS  OF  THE  ATOMS.  159 

part  of  this  time  oxygen  was  rounded  off  into  the  whole 
number  16,  and  for  the  remainder  varied  very  little  from 
this  figure,  it  is  evident  that  the  value  of  the  literature 
of  this  period  will  be  least  impaired  by  the  adoption  of 
oxygen  as  16.  The  labor  of  learning  a  new  table  and  of 
converting  the  old  data  into  the  new  system  would  be 
both  burdensome  and  distasteful  if  any  other  number 
were  chosen.  At  the  same  time  this  is  the  smallest  num- 
ber which  can  be  assigned  oxygen,  still  keeping  all  other 
atomic  weights  greater  than  unity. 

The  discussion  has  been  a  prolonged  one  and  is  not 
entirely  settled  yet,  although  the  first  century  of  the 
atomic  weights  will  soon  draw  to  its  close.  The  discus- 
sion is  not  as  to  the  theory  but  involves  such  simple 
questions  that  it  should  have  been  satisfactorily  settled 
long  ago. 


CHAPTER  V. 


The  Periodic  or  Natural  System, 


CHAPTER  V. 
THE  PERIODIC  OR  NATURAL  SYSTEM. 

The  grouping  together  of  chemical  bodies  according  to 
certain  observed  analogies,  was  attempted  before  ele- 
ments were  distinguished  from  compounds,  and,  as  has 
been  pointed  out,  numerical  relationships  were  suggested 
between  combining  equivalents  before  the  new  atomic 
theory  had  been  formulated.  Such  relationships  then 
need  not  have  any  bearing  upon  the  question  of  the  atoms 
which  forms  our  immediate  study.  But  very  soon  after 
Dal  ton's  announcement  of  the  atomic  theory  some  of  these 
relationships  were  regarded  as  revealing  the  possibly  com- 
posite nature  of  the  elementary  atoms,  and  from  the  study 
of  these  regularities  and  analogies  there  has  been  gradu- 
ally unfolded  that  which  has  been  called  the  Periodic  Sys- 
tem of  the  Elements,  but  for  which  the  name  Natural 
System,  first  assigned  to  it,  might  with  great  advantage 
be  again  adopted. 

The  discovery  of  this  Natural  System  has  done  so  much 
to  make  clearer  the  nature  of  the  atom  that  a  careful  study 
of  its  development  and  characteristics  is  most  essential. 
Very  little  space  need  be  given  to  the  many  efforts  at  dis- 
covering numerical  regularities  between  the  atomic 
weights,  as  they  have  thrown  little  light  upon  the  atom, 
and  have  not  succeeded  in  proving  its  divisibility  nor 
composite  nature.  Most  of  the  regularities  require  the  ac- 
ceptance of  approximations  instead  of  rigidly  adhering  to 
actually  determined  numbers ;  many  of  the  deductions 
have  little  basis,  and  the  simple  arithmetical  probability 
of  many  such  regularities  occurring  between  any  70  num- 
bers chosen  at  random,  ranging  from  i  to  240,  does  not 
seem  to  have  been  taken  into  account. 


164  A  STUDY  OF  THE  ATOMS. 

Historically,  the  most  noted  of  these  regularities  is  that 
which  has  become  famous  as  Prout's  hypothesis. 

In  1815,  1  in  a  paper  upon  the  relations 


Yi       th     *  between  the  specific  gravities  of  bodies 

in  the  gaseous  state  and  their  atomic 
weights,  Prout  stated  that  he  had  of  ten  observed  the  near 
approach  to  round  numbers  of  many  of  the  weights  of  the 
atoms.  From  the  table  at  his  command  he  further  de- 
duced that  all  elementary  numbers,  hydrogen  being  con- 
sidered as  i,  are  divisible  by  4,  except  carbon,  nitrogen 
and  barium,  and  these  are  divisible  by  2,  appearing,  there- 
fore, to  indicate  that  they  are  modified  by  a  higher  num- 
ber than  unity  or  hydrogen.  He  thought  the  other  num- 
ber might  be  1  6  or  oxygen,  and  that  possibly  all  sub- 
stances were  composed  of  these  two  elements. 

L,ater,  in  i8i6,2  he  expressed  the  following  views  :  "  If 
the  views  we  have  ventured  to  advance  be  correct,  we  may 
almost  consider  the  npoorrf  v\rj  of  the  ancients  to  be  re- 
alized in  hydrogen,  an  opinion,  by-the-by,  not  altogether 
new.  If  we  actually  consider  this  to  be  the  case,  and 
further  consider  the  specific  gravities  of  bodies  in  their 
gaseous  states  to  represent  the  number  of  volumes  con- 
densed into  one,  or,  in  other  words,  the  number  of  the 
absolute  weights  of  a  single  volume  of  the  first  matter 
(npGorrj  vkrj}  which  they  contain,  which  is  extremely 
probable,  multiples  in  weight  must  always  indicate  mul- 
tiples in  volume  and  vice  versa,  and  the  specific  gravities 
or  absolute  weights  of  all  bodies  in  a  gaseous  state  must 
be  multiples  of  the  specific  gravity  or  absolute  weight  of 
the  first  matter,  because  all  bodies  in  a  gaseous  state, 
which  unite  with  one  another,  unite  with  reference  to  this 
volume.  '  ' 

1  Ann.  Phil.,  Thomsen,  1815,  n,  321. 

2  Ann.  Phil.,  Thomsen,  1816,  12,  HI. 


THE  PERIODIC  OR   NATURAL  SYSTEM.  165 

Now  this  is  all  of  the  evidence  and  the 
Berzelius'  only  argument  which  has  ever  been 

adduced  in  favor  of  this  hypothesis. 
And  yet  the  fascinating  dream,  a  kind  of  renascence  of 
the  Pythagorean  belief  in  the  unity  of  matter,  was  pur- 
sued as  an  ignis  fatuus  throughout  the  igth  century. 
Several  times  it  was  thought  to  have  been  disproved  and 
the  question  satisfactorily  settled,  but  after  a  brief  disap- 
pearance it  came  forth  again,  sometimes  in  a  modified 
form  and  with  new  followers.  Its  first  and  strongest  an- 
tagonist was  Berzelius  who,  however,  had  regarded  it 
with  favor  when  first  brought  to  his  notice.  In  1825  he 
published  a  table  of  the  atomic  weights  which  contained 
a  number  of  fractions,  and  he  protested  very  strongly 
against  the  practice  of  rounding  off  these  fractions  into 
whole  numbers.  As  Hoffman  says,  ' '  He  could  not  per- 
suade himself  that  the  numerical  relations  of  these  values 
betokened  an  inner  connection  of  the  elements  nor  yet  a 
common  origin.  On  the  contrary,  he  was  of  the  opinion 
that  these  apparent  relations  would  disappear  more  and 
more  as  these  values  were  more  accurately  determined. 
For  him,  therefore,  there  existed  as  many  forms  of  matter 
as  there  were  elements  :  in  his  eyes  the  molecules  of  the 
various  elements  had  nothing  in  common  with  one 
another  save  their  immutability  and  their  eternal  exis- 
tence." Our  later  knowledge  of  these  matters  would 
seem  to  show  that  in  this  Berzelius  had  gone  too  far  to 
the  other  extreme. 

In  1832  Turner  was  specially  dele- 
Fate  of  ted  b  tlie  British  Association  to 
the  Hypothesis.  .  ,.  .  _r,  . 

investigate  this  question.    If  barium, 

chlorine,  etc.,  really  had  fractional  atomic  weights,  then 
the  hypothesis  in  its  original  form  was  untenable.     Tur- 


1 66  A  STUDY  OF  THE  ATOMS. 

ner's  results  were  adverse  to  it.  So  also  were  Penny's. 
Marignac  suggested  that  if  half  the  atomic  weight  of  hy- 
drogen were  taken,  then  all  known  atomic  weights  would 
be  practically  multiples  of  it.  The  idea  was  taken  up  by 
Dumas  with  enthusiasm,  but  he  found  this  factor  must 
be  once  more  halved  and  thus  one-fourth  the  hydrogen 
atom  taken.  It  is  not  quite  clear  why  this  is  not  a  beg- 
ging and  abandonment  of  the  whole  question.  But  the 
very  careful  and  accurate  work  of  Stas  upon  the  atomic 
weights  made  even  this  position  impossible.  When  Zan- 
gerle1  extended  the  hypothesis  to  the  o.ooi  part  of  the 
hydrogen  atom  it  passed  the  limit  of  all  experimental  evi- 
dence and  lost  all  weight  and  meaning.  Accurate  deter- 
minations have  shown  that  while  certain  of  the  atomic 
weights  approximate  closely  to  whole  numbers,  others 
usually  do  not.  The  hydrogen  atom  cannot  be  contained 
in  them  an  even  number  of  times.  Any  fraction  what- 
ever of  the  weight  of  the  hydrogen  atom  cannot  be  con- 
sidered without  abandoning  the  fundamental  idea  of  an 
atom  and  such  consideration  can  have  no  clear  meaning 
nor  be  of  any  true  value. 

There  have  been  a  number  of  attempts 

at  discovering  some  mathematical  for- 
Regulanties.  .    ,  .  ,  .        .  _  A 

mula  by  means  of  which  atomic  weights 

might  be  calculated  or  interpolated  in  a  series.  Thus 
there  was  the  equation  of  Cooke2  elaborated  still  further 
by  Dumas3  ;  the  logarithmic  expression  of  Johnstone 
Stoney*  ;  the  algebraic  expression  of  Carnelley5,  and 
others  equally  futile.  This  truth  is  made  apparent  by 
the  most  accurate  determinations  that  these  atomic 

1  Ber,  d.  chem.  Ges.,  4,  570-574. 

2  Am.J.  Set.,  [2],  17,  387- 

3  Compt.  Rend.,  45,  709  ;  46,  951;  47,  1026. 
*  Chem.  News,  57,  163. 

5  Phil.  Mag.  (5),  29,  97-115- 


THE  PERIODIC  OR  NATURAL  SYSTEM.  167 

weights  do  not  form  a  regular  series  but  a  most  irregular 
one,  the  gradations  from  one  to  the  other  varying  too 
greatly  to  meet  the  requirements  of  any  mathematical 
expression.  If  there  is  any  deeper  meaning  in  the  great 
number  of  numerical  regularities  observed  it  has  not  been 
discovered. 

The  first  classification  of  the  elements 
Depberemer's  depending  upon  the  atomic  weights 

was  through  what  were  known  as  the 
Triads  of  Dobereiner.  This  chemist  seems  to  have 
observed1  first  that  the  combining  weight  of  strontium 
was  the  arithmetical  mean  of  those  of  calcium  and  barium. 
This  was  in  1816,  and  the  accepted  numbers  at  that  time 
were  27.5  for  calcium,  72.5  for  barium  and  50  for  stron- 
tium. This  led  him  for  a  while  to  question  the  independ- 
ent existence  of  strontium.  After  the  publication  in  1825 
of  the  more  accurate  table  of  atomic  weights  by  Berzelius, 
the  matter  was  brought  up  again  by  Dobereiner.  Several 
such  triads  were  mentioned,  as  lithium,  sodium,  and 
potassium  ;  chlorine,  bromine  and  iodine  ;  sulphur,  sele- 
nium, and  tellurium.  He  was  careful  not  to  let  this  group- 
ing depend  upon  the  atomic  weights  alone  but  insisted 
that  only  elements  exhibiting  decided  analogies  of  proper- 
ties must  be  considered  together.  Thus  the  fact  that 
nitrogen  was  the  mean  between  carbon  and  oxygen  could 
not  be  held  as  meaning  anything  since  no  analogy  ex- 
isted between  them.  Such  warning  was  most  clearly 
needed  for  it  is  evident  that  wherever  the  atomic  weight 
of  an  element  happened  to  be  equidistant  from  any  other 
two  it  would  form  the  arithmetic  mean.  Of  course  there 
would  be  a  large  number  of  such  groups.  It  is  evident 
that  to  generalize  on  this  slight  evidence  so  as  to  deduce 

1  Ann.  d.phys.,  56,  332. 


1  68  A  STUDY   OF  THE  ATOMS. 

a  supposed  law  that  the  elements  occurred  in  groups  of 
threes,  is  going  to  an  unwarranted  length,  yet  this  seems 
to  be  the  assumption.  It  was  taken  up  by  other  chem- 
ists, notably  by  Gmelin  in  his  "  Handbook,"  and  many 
analogies  and  groups  were  sought  for.  Then  for  a  number 
of  years  little  attention  was  paid  to  these  triads.  In  1857, 
however,  Lennsen1  returned  to  this  doctrine  of  triads, 
endeavoring  to  force  all  of  the  elements  into  some  twenty 
such  groups.  Then  Odling2  endeavored  to  build  upon 
them  an  elaborate  system  of  the  elements  which  he  called 
the  Natural  System.  The  system  was  also  based  on  a 
consideration  of  all  known  properties  as  well  as  the  atomic 
weights.  It  was  too  artificial  and  faulty  to  receive  much 
attention. 

These  triads  of  elements  can  still  be  seen  in  the  more 
perfect  system  of  to-day,  but  the  interpretation  of  the 
phenomenon  is  as  far  off  as  ever.  It  should  be  noted 
that  the  fact  that  the  atomic  weight  is  the  arithmetical 
mean  of  those  of  two  other  analogous  elements,  carries 
with  it  often  the  further  phenomenon  that  the  other 
properties  are  arithmetical  means  also.  In  this  we  can 
only  see  one  of  the  fundamental  propositions  of  the  periodic 
system. 

The  first  to  suggest  an  arrange- 
Gladstone's  Ascend-        ment     f  ^     elements  in    the 

ing  Series.  .  .          . 

order  of  their  atomic  weights 

was  Gladstone.3  This  was  in  1853  and  he  made  use  of 
the  faulty  and  imperfect  table  of  weights  given  in  Liebig's 
Jahresberichte  for  1851.  Thus  the  atomic  weights  of 
metals  analogous  to  iron  were  halved.  This  threw  a  large 
number  of  elements  as  aluminum,  silicon,  chromium, 

1  Ann.  Chem.  Pharm.,  103,  121. 


*/%i7.  Mag.,  U],  5,313. 


THE  PERIODIC  OR  NATURAL  SYSTEM.  169 

manganese,  iron,  cobalt  and  nickel  between  the  numbers 
27  and  29.  This  and  other  groups  having  nearly  the 
same  atomic  weight  for  a  number  of  elements  attracted 
the  attention  of  Gladstone  and  misled  him,  obscuring  the 
natural  system  of  the  elements.  It  is  not  surprising  then 
that  his  Ascending  Series  received  little  notice. 

Several  ingenious  observations  and  sug- 
3°™*  gestions  were  made  during  this  period 

immediately  preceding  the  introduction 
of  the  revised  atomic  weights.  Thus  Pettenkofer1  com- 
pared the  elements  with  the  compound  radicals  of  organic 
chemistry  and  suggested  that  they  might  be  looked  at 
from  the  same  standpoint.  Later,  Dumas,2  making  use 
of  the  formula  devised  by  Cooke,  tried  to  reproduce  with 
the  elements  homologous  series  similar  to  those  of  the 
organic  radicals.  L,ater  still.,  there  were  one  or  two  efforts 
at  arranging  the  elements  according  to  the  lately  discov- 
ered property  of  valence. 

The  first  use  of  the  revised  atomic  weights  in 
e  uric         an  ascending  series  was  by  De  Chancourtois,3 

and  though  his  work  lay  unnoticed  for  thirty 
years,  it  contained  much  of  the  Periodic  L,aw.  He  drew 
as  a  conclusion  from  his  work  that :  ' '  L,es  proprietes  des 
corps  sont  les  proprietes  des  nombres. ' ' 

The  fundamental  idea  of  the  Telluric  Screw  consisted 
in  writing  the  values  of  the  atomic  weights  along  the  gen- 
eratrix of  a  vertical  cylinder,  the  circular  base  of  which 
was  divided  into  16  equal  parts,  16  being  the  atomic  weight 
of  oxygen.  If  we  then  trace  upon  the  cylinder  a  helix 
with  an  angle  of  45°  to  its  axis,  each  point  of  the  helix 

1  Ann.  Chtm.  Pharm.,  105,  188. 

2  Com.pt.  rend.,  45,  709. 

3  "Vis  Tellurique,"  Paris,  1863. 


170  A  STUDY  OF  THE  ATOMS. 

may  be  considered  as  the  characteristic  point  of  a  simple 
body,  the  atomic  weight  of  which,  proportional  to  the  cor- 
responding length  of  the  spiral,  will  be  read  upon  the 
generatrix  which  passes  by  this  point.  At  each  turn,  the 
helix  returns  on  one  and  the  same  perpendicular  at  dis- 
tances from  the  summit  of  the  cylinder,  which  are  multi- 
ples of  1 6,  and  mark  the  bodies  whose  atomic  weights 
conform  to  this  condition.  In  the  same  manner  the 
various  points  of  intersection  of  the  helix  with  any  of  the 
sixteen  principal  generatrices,  traced  from  the  divisions 
of  the  circular  base,  correspond  to  elements  whose  atomic 
weights  differ  among  themselves  by  16  or  a  multiple  of  16. 
We  have  in  this  arrangement  evidences  of  the  influence 
of  Dumas,  especially  in  the  emphasis  laid  upon  the  num- 
bers 8  and  16.  It  is  manifest  also  that  De  Chancourtois 
started  out  with  the  idea  that  the  differences  between  the 
atomic  weights  ought  to  be  constant.  Gaps  were  filled  out 
by  imagining  new  varieties  of  known  simple  bodies  which 
he  called  Secondary  Characters.  Analogies  were  forced, 
and  there  were  other  faults  which  prevented  a  wide  con- 
sideration or  acceptance  of  the  arrangement. 

In  the  work  of  Newlands,1  which  followed 
O  ta  °  closely  upon  that  of  De  Chancourtois,  we 

we  have  a  nearer  approach  to  the  Natural 
System.  He  too  arranged  the  elements  in  an  ascending 
series  according  to  their  atomic  weights.  Numbering 
these  elements  i,  2,  3,  etc.,  he  observed  that  the  differ- 
ence between  the  number  of  the  lowest  member  of  a  group 
and  that  immediately  above  it  is  7  ;  in  other  words,  the 
eighth  element  starting  from  a  given  one  is  a  kind  of  repe- 
tition of  the  first,  like  the  eighth  note  in  music.  But 
then  he  lost  his  grasp  of  the  system,  maintaining  that  the 
differences  between  the  numbers  of  the  other  members  of 

1  Chem.  News,  10,  94. 


THE   PERIODIC   OR  NATURAL  SYSTEM.  171 

a  group  are  frequently  twice  as  great ;  thus  in  the  nitrogen 
group,  between  nitrogen  and  phosphorus  there  are  7  ele- 
ments ;  between  phosphorus  and  arsenic  13 ;  between  ar- 
senic and  antimony  14 ;  and  between  antimony  and  bis- 
muth 14.  The  truth  is,  the  list  of  atomic  weights  was 
still  too  imperfectly  filled  out  for  the  system  to  appear 
clearly,  or  for  one  to  grasp  it,  unless  iiirnished  with  a  wide 
knowledge  of  chemical  facts. 

A  year  later1  Newlands  had  still  further  worked  out 
his  idea,  giving  his  discovery  the  name  of  ' '  a  law. ' '  In 
his  new  table  which  is  here  reproduced,  he  transposed 
some  of  the  elements  so  as  to  bring  them  into  their  proper 
groups.  Arranging  his  table  in  a  vertical  series,  he  ob- 
served that  elements  belonging  to  the  same  group  usually 
appear  on  the  same  horizontal  line.  In  order  to  allow  for 
certain  elements  which  had  their  atomic  weights  very 
close  together,  as  cobalt  and  nickel,  Newlands  modified 
his  law  thus :  ' '  The  numbers  of  analogous  elements  when 
not  consecutive  differ  by  7  or  by  some  multiple  of  7." 

NEWTVAND'S  TABLE  OF  OCTAVES  (1866). 

No.  No.  No.  No. 

H i  F..  8  Cl 15  CoandNi22 

14 2  Na.  9  K 16  Cu 23 

G 3  Mg  10  Ca 17  Zn 24 

Bo 4  Al.  ii  Cr 18  Y 25 

C 5  Si  •  12  Ti  19  In 26 

N 6  P..  13  Mn 20  As 27 

0 7  S-.  14  Fe 21  Se 28 

Br   29  Pd.   36           Te 43  Pt  and  Ir.  50 

Rb 30  Ag.  37            Cs 44  Os 51 

Sr 31  Cd.  38  BaandV45  Hg 52 

Ce  and  La.  32  U- .  39           Ta 46  Tl 53 

Zr 33  Sn-  40            W 47  Pb 54 

DiandNeo34  Sb-  41            Nb 48  Bi 55 

Ro  and  Ru  35  I  . .  42            Au 49  Th 56 

1  Chem.  News,  12,  83. 


172  A   STUDY  OF  THE   ATOMS. 

In  1866  Hinrichs1  deduced  from  his 
Hinrichs  on  the         observations  on  the  spectra  of  ele. 

r  rOpcillcS. 

ments  the  important  fact  that  the 

properties  of  the  chemical  elements  are  functions  of  their 
atomic  weights.  This  was  three  years  before  the  announce- 
ment of  Mendeleeff  and  the  modes  of  expression  are 
almost  identical. 

The  name  of  L,othar  Meyer  has  been  very 
T  ebier  *  commonly  associated  with  the  development 

of  the  Periodic  or  natural  system.  His  first 
table  was  given  in  1864  in  the  first  edition  of  his  work 
"  Die  modernen  Theorien  der  Chemie."  The  table  was 
as  follows : 

MEYER'S  FIRST  TABLE,  1864. 


4  Val. 

3  Val. 

2  Val. 

i 

Val. 

i  Val. 
Li...     7.03 

2  Val. 
(Be..      9.3) 

Diff, 

,  



.... 

.... 

16.02 

(14.7) 

C  .. 

I2.O 

N.. 

14.04 

O..     16.00 

F.. 

19.0 

Na. 

.   23.05 

Mg..    24.0 

Diff. 

I6.5 

16.96 

16.07 

16.46 

16.08 

16.0 

Si.. 

28.5 

p  .. 

31-0 

S  ..    32.0 

Cl. 

35.46 

K.. 

•  39.13 

Ca  .  .    40.0 

Diff. 

44-45 

44.0 

46.7 

44-51 

46.3 

47.0 

.... 

As  . 

75-0 

Sc  .    78.8 

Br  . 

79-97 

Rb. 

•  85.4 

Sr...    87.0 

Diff. 

44-55 

45-6 

49-5 

46.8 

47.6 

49.0 

Sn. 

117.6 

Sb. 

I2O.6 

Te.  128.3 

I... 

126.8 

Cs. 

133-0 

.... 

Diff. 

44-7 

43-7 

.... 



35-5 

.... 

Pb. 

207.0 

Bi.. 

208.0 





Te. 

(204.0) 

Ba.  ..137.1 

4 

Val. 

4 

Val. 

4  Val. 

2  Val. 

i  Val. 

fMn. 

55-i 

Ni 

58-7 

Co.    58.7 

Tn. 

65.0 

Cn. 

•-  63.5 

IF.. 

56.0 

.... 

Diff. 

48^3 

45-6 

47.3 

46-9 

44.4 

Rn. 

104.3 

Rh. 

104.3 

Pd.  106.0 

Cd. 

111.9 

Ag 

.107.94 

Diff. 

46.0 

46.4 

46.5 

44-5 

44-4 

Pt.. 

197.1 

Ir.. 

197.1 

Os  .  199.0 

Hg 

200.2 

An 

.196.7 

lAm.J.  Set.,  32,  350. 


THE   PERIODIC  OR   NATURAL  SYSTEM.  173 

The  elements  are  arranged  horizontally  in  the  sequence 
of  their  atomic  weights  to  a  certain  extent,  but  a  number 
of  elements  as  copper,  silver,  gold,  and  others,  were  ex- 
cluded from  their  proper  sequence.  It  is  clear  from  a 
closer  study  of  the  table  that  the  idea  of  the  natural  fami- 
lies, already  well  known,  was  the  predominant  one  and 
that  the  numerical  order  of  the  atomic  weights  was  sub- 
ordinated to  it.  Thus  the  four  first  elements  form  a 
series,  and  the  others  are  in  sixes.  Some  elements  are 
omitted  and  vacant  spaces  are  left  in  other  cases.  In  the 
fourth  series,  we  have  the  first  member  omitted  in  order 
that  analogous  elements  may  fall  properly.  There  is  less 
evidence  of  periodicity  than  in  the  table  of  Newlands. 

His  incomplete  table,  as  handed  to  Remele  in  I8681,  is 
but  a  slight  improvement  over  the  earlier  table  and  still 
shows  only  a  groping  after  the  cardinal  principles  of  the 
system,  namely,  the  orderly  sequence  of  the  weights  and 
the  periodicity  of  the  elements.  L,ater,  in  1870,  one  year 
after  the  publication  of  Mendeleeff's  table,  Meyer  gave  a 
third  table  in  which  the  order  of  the  1864  table  is  re- 
versed, the  sequence  falling  in  vertical  lines  instead  of 
horizontal.  This  latter  table  indeed  bears  scant  resem- 
blance to  either  of  the  earlier  ones.  As  Meyer  had  seen 
an  abstract  of  Mendele'efPs  article  before  the  publication 
of  his,  he  stated  later  that  he  claimed  credit  only  for 
points  in  which  he  thought  he  had  improved  upon  that 
table.  Taking  all  into  consideration,  it  is  difficult  to  fix 
upon  any  important  contribution  of  Meyer  to  the  discov- 
ery of  the  Periodic  Law. 

The  first  table  published  by  Mendeleeff2 

Mendeleefi          *s  one  w^^  a  vertical  arrangement  ac- 
cording to  atomic  weights,  a  second  ta- 

1  Ztschr.  anorg.  Chem.,  9,  354. 

2  J.  Russ.  Chem.  Soc.,  1869,  p.  90. 


174  A   STUDY   OF  THE   ATOMS. 

ble,  however,  accompanying  it,  which  showed  much  more 
clearly  the  natural  system.  The  first  gave  all  the  ele- 
ments, with  blank  spaces  for  four  unkown  elements.  It 
failed  very  decidedly  to  show  the  natural  families  of  ele- 
ments except  in  the  case  of  a  few  well-known  ones.  The 
table  is  here  given  : 

MENDBI,EEFF'S  TABI,E,  1869. 

Ti 50     Zr ..    90     ?         180 

V 51      Nb..     94     Ta..   182 

Cr  ....  52      Mo  .     96     W  ..  186 
Mn  ...  55      Rh..  104.4  Pt..  197.4 

Fe 56      Ru..  104.4  Ir...   198 

NiCo..  59      Pd..   106.6  Os..  199 

H-.  i  Cu«...  63.4  Ag..  108     Hg..  200 

Be-.     9.4  Mg..  24      Zn....  65.2  Cd..  112 
B...  ii      Al  ••  27.4?  68      Ur..  116      Au..  197 

C...  12      Si  ..  28      ?  70      Sn..  118 

N  . .   14      P  ...  31      As  ....  75      Sb  . .  122      Bi  • .  210 

O...  16     S...  32     Se 79     Te  ..  128 

F...  19  Cl  ..  35.5  Br  ....  80  I....  127 
Na..  23  K...  39  Rb....  85.4  Cs  -.133  Tl  ..  204 
Ca  ..  40  Sr  ....  87.0  Ba  ..  137   Pb..  207 
?    45   Ce....  92 
?    56  La....  94 

?    60  Di 95 

?    75.6  Th....n8 

The  second  table  did  not  include  all  of  the  elements  and 
had  many  blank  spaces.  Evidently  the  author  was  very 
seriously  handicapped  by  the  imperfect  knowledge  of  a 
large  number  of  elements  then  at  his  command.  Nothing 
can  better  illustrate  the  immense  service  which  this  sys- 
tem has  been  to  the  science  of  chemistry  than  the  in- 
crease in  knowledge  of  these  elements  and  their  com- 
parative properties  since  that  time.  Much  of  this  increase 
can  be  directly  traced  to  the  influence  of  this  great  dis- 
covery. 


THE  PERIODIC  OR  NATURAL  SYSTEM.  175 

MENDEI,EEFF'S  SECOND  TABI,E. 

Li        Na  K         Cu        Rb  Ag  Cs         ..        Tl 

Be       Mg  Ca        Zn        Sr  Cd  Ba         ..        Pb 

B         Al  Ur  ..          ..        Bi 

C         Si  Ti          ..          Zr  Sn  

N         P  V          As        Nb  Sb  ..         Ta       .. 

OS  Se        ..  Te  ..         W 

F         Cl  ..          Br         ..  I  

It  required  an  insight  into  the  principles  of  the  natural 
system  to  devise  this  latter  table,  and  the  conclusions 
reached  by  Mendeleeff  in  this  first  paper  give  proof  that 
he  had  grasped  these  principles.  The  most  important  of 
these  were  : 

1 .  The  elements,  if  arranged  according  to  their  atomic 
weights,  exhibit  an  evident  periodicity  of  properties. 

2.  Elements  which  are  similar  as  regards  their  chemi- 
cal properties  have  atomic  weights  which  are  either  of 
nearly  the  same  value  or  which  increase  regularly. 

3.  The  arrangement  of  the  elements,  in  the  order  of 
their  atomic  weights,  corresponds  to  their  so-called  va- 
lences, as  well  as,   to  some  extent,  to  their  distinctive 
chemical  properties. 

4.  The  elements  which  are  most  widely  diffused  have 
small  atomic  weights. 

5.  The  magnitude  of  the  atomic  weight  determines  the 
character  of  the  element  just  as  the  magnitude  of  the 
molecule  determines  the  character  of  a  compound  body. 

Two  years  later  he  gave  a  table  which  is  practically  the 
same  in  form  as  the  one  in  use  at  the  present  day. 

Mendeleeff  especially  emphasized  the  idea  of  periodic- 
ity. Afterwards  he  said  i1  "  The  repetition  of  the  word 
periodicity  shows  that  from  the  very  beginning  I  held  this 
to  be  the  fundamental  property  of  my  system  of  the  ele- 

1  Ber.  d.  chem.  Ges.,  13,  1796. 


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THE   PERIODIC   OR   NATURAL   SYSTEM.  177 

ments. ' '  Further,  he  maintained  that  the  system  can  be 
arranged  in  the  form  of  a  spiral  and  in  this  the  resem- 
blances appear  among  the  members  of  every  other  series. 
Mendele"eff  devoted  great  energy  and  wide  chemical 
knowledge  to  the  filling  out  of  his  great  discovery,  and 
the  credit  for  the  expansion  and  filling  out  of  the  system, 
and  the  bringing  of  various  compounds  of  the  elements 
into  consideration  also  has  been  largely  due  to  his  skill 
and  knowledge. 

It  is  evident  that  any  classification  of 

the  elements  which  Purports  to  be  in 
accord  with  nature,  or  a  natural  sys- 
tem, cannot  have  an  arbitrary  basis,  but  must  be  on  one 
or  more  of  the  properties  of  the  elements.  It  is  conceiv- 
able that  the  selection  of  different  properties  as  bases 
might  lead  to  varying  systems.  The  Periodic  System 
deserves  the  name  of  the  Natural  System  par  excellence 
because  it  is  not  based  on  one  but  all  of  the  properties  of 
the  elements.  There  were  a  number  of  efforts  at  classi- 
fying these  elements  before  the  discovery  of  this  system, 
but  all  were  unsatisfactory.  Thus  the  division  into 
metals  and  non-metals,  or,  as  they  were  called,  metalloids, 
was  in  the  main  based  upon  the  electrochemical  character. 
The  division  into  artiads  and  perissads  depended  upon 
the  valence.  A  more  common  grouping  was  into  families 
of  analogous  elements.  Here  analogies  of  properties  in 
general  were  made  use  of.  This  enabled  a  few  strongly 
marked  groups  to  be  distinguished,  but  a  large  number 
of  the  elements  could  not  thus  be  grouped  and  the  system 
was  only  a  partial  one  and  not  a  complete  classification. 
A  proper  classification  conduces  greatly  to  systematic  and 
successful  study,  and  the  lack  of  this  accounts  for  the 


178  A  STUDY  OP  THE  ATOMS. 

slow  development  of  inorganic  chemistry  before  the  an- 
nouncement of  the  Periodic  System. 

Periodicity  of        °°  exa°f  inS  the  I*™1*  .<*  Cements 
Properties.  obtained  by  arranging  them  in  an  ascend- 

ing series  according  to  their  atomic 
weights  it  is  readily  seen  that  this  accords  with  the  electro- 
chemical properties  also,  for  each  period  begins  with  a 
positive  element  and  the  positive  character  diminishes 
until  at  the  end  of  each  period  is  a  negative  element.  In 
these  periods  again,  the  valence  in  regard  to  hydrogen 
increases  regularly  up  to  the  fourth  member,  and  then 
regularly  diminishes  until  the  seventh  member  is  reached. 
The  oxygen- valence  increases  regularly  from  the  first  to 
the  seventh  member.  Similar  gradations  are  noted  in 
the  case  of  other  properties  such  as  specific  gravity, 
specific  heat,  solubility,  melting-point,  etc.  Thus  this 
natural  system  should  be  looked  upon  as  based  upon  a 
consideration  of  all  the  properties  and  not  upon  the  weight 
of  the  atomic  masses  alone.  This  point  of  view  is  an  im- 
portant one  and  should  be  borne  in  mind.  The  same 
arrangement  could  have  been  arrived  at,  independent  of 
the  atomic  weights,  by  a  consideration  of  the  valence  or 
electrochemical  character  alone. 

It  is  very  important  for  the  study  and 

resentation?  proper  SrasPinS  of  the  Periodic  system 
that  a  suitable  graphic  representation 
of  it  be  devised.  There  have  been  many  attempts  at 
doing  this,  most  of  which  have  failed  and  none  of  which 
have  proved  entirely  satisfactory.  There  are  certain 
essential  points  to  be  considered  in  devising  any  such 
graphic  table.  First,  the  periods  must  be  properly  given. 
These  may  be  looked  upon  as  periods  of  7  with  periods  of 
3  occurring  at  intervals.  Some  have  chosen  to  consider 


THE  PERIODIC  OR  NATURAL  SYSTEM.  179 

them  as  periods  of  7  and  17.  Again,  the  analogies  of 
the  elements  must  be  kept  in  mind  and  should  appear 
clearly  in  the  scheme.  Then  the  differences  between  the 
atomic  weights  of  the  elements  in  the  ascending  series 
must  be  considered.  These  may  be  called  the  atomic 
weight  differences.  lastly,  the  distances  between  the 
periods,  that  is,  the  differences  between  the  atomic  weights 
of  analogous  elements  in  adjoining  periods,  have  an  im- 
portant bearing  upon  the  arrangement.  It  must  be  borne 
in  mind  that  the  atomic  weight  differences  and  the  period 
distances  are  far  from  uniform.  The  first  have  a  range 
from  i  to  6  or  more,  and  the  latter  from  15  to  51  or  possi- 
bly 91.  This  has  been  lost  sight  of  by  many  who  have 
grappled  with  the  problem.  It  should  be  perfectly  clear 
that  it  renders  any  tracing  of  a  regular  curve  absolutely 
impossible.  Many,  as  de  Chancourtois,1  Meyer,2  Baum- 
hauer,3  Huth*  and  others  have  fancied  the  spiral  arrange- 
ment or  that  of  a  helix.  Indeed  Mendeleeff  suggested 
this  also.  No  spiral  can  be  devised,  however,  Archime- 
dean, logarithmic  or  of  any  other  character,  which  will 
allow  for  the  irregularities  in  both  atomic  weight  differences 
and  period  distances.  All  who  attempted  a  spiral  repre- 
sentation have  disregarded  this  lack  of  uniformity  and 
rested  content  with  crude  approximations  which  cannot 
be  tolerated.  If  such  a  mathematical  curve  could  be 
drawn,  manifestly  the  equation  to  it  would  give  a  simple 
and  easy  method  of  calculating  any  and  all  atomic  weights 
with  great  accuracy.  In  spite  of  exhaustive  search,  such 
an  equation  has  never  been  found  and  such  calculations 
have  not  proved  possible. 

1  Loc.  cit. 

2  Meyer  :  "  Mod.  Theorien  d.  Chemie." 

1  "  Bez.  zwisch.  d.  Atomgew."  Braunschweig,  1870. 


'  Period.  Gesetz  d.  Atomgew.,"  Frankfurt,  1884. 


i8o 


A   STUDY   OF   THE   ATOMS. 


Three  tables  have  been  given  which  have  been  more  or 
less  used  and  which  present  points  of  advantage  over 
most  others.  These  tables  follow  : 

TABLE  OF  LOTHAR  MEYER. 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

14 

7.08 

Be 

B 
10.9 

c 
11.97 

N 
14.01 

0 

F 
19.06 

Na 
22.99 

K 

Cu 
63.18 

Rb 
J5^2_ 

107.66 

Cs 
132.7 

? 

165 

An 

Mg 

Al 
27.04 

Si 
28 

P 
30.96 

Ca 

s 

Sc 
43-97 

Cl 

35-37 

Ti 
48 

V 
5I.I 

Zn 

Cr 
52.45 

Se 
78.87 

Ga 
69.9 

Mil 
54.8 

Ger 
72 

Fe          Co          Ni 
55.86     58.6      58.6 

As 
74-9 

Sr 
87.3 

Y 
89.6 

In 

Br 
79.76 

Zr 
90.4 

Nb 
93.7 

Sb 
II9.6 

Cd 

111.7 

Mo 
95.9 

Te 
126.3 

? 
99 

Sn 
H7.35 

Ce 
141.2 

Ru         Rh          Pd 
103.5    104.1    106.2 

Ba 

!38.5 

i 
126.54 

Di 

170 

f 

Yb 
172.6 

Tl 
203.7 

152 

I76 

Ta 

Os           Ir           Pt 
191       193.5     194.3 

Hg 
199.8 

W 
183.6 

185 

Pb 
206.  39 

222 

226 

Bi 
207.5 

210 

23° 

211 

Th. 

234 

Ur 
239.8 

THE  PERIODIC  OR  NATURAL  SYSTEM. 


BAYI,BY'S  TABI,E,  1882. 


Te 
I 


Th 

-U 


Ru 

—           Os 

Rh 

—            Ir 

Pd 

—           Pt 

•Ag 

—           Au 

-Cd 

Hg 

-In 

Tl 

^n 

—           Pb 

-Sb 

—           Bi 

Phil.  Mag.,  (5),  13,  26. 


182 


A   STUDY   OF  THE   ATOMS. 


THE   PERIODIC   OR   NATURAL  SYSTEM.  183 

The  closer  study  of  the   system  has 

uls  °f  shown  that  the  elements  of  the  first 

the  Elements.  .    ,  ,     .         —, 

period  present  many  analogies.    Thus 

lithium  approaches  beryllium  in  many  of  the  character- 
istics, boron  resembles  carbon  so  much  that  in  earlier  classi- 
fications it  was  placed  in  the  same  family.  The  character- 
istics of  the  group  appear  more  distinctly  in  the  elements 
of  the  second  period.  Thus  sodium  is  more  typically  an 
alkali  than  lithium,  magnesium  more  representative  of  its 
group  than  oxygen,  phosphorus  more  of  its  group  than 
nitrogen,  etc.  It  would  seem  that  the  groups  diverged 
more  and  more  in  their  properties  as  the  atomic 
weights  increased,  or  as  they  got  further  away  from  some 
common  origin.  The  cross  analogies  are  prominent  in 
the  first  period,  but  less  so  in  the  second,  and  hence  have 
less  effect  in  modifying  the  development  of  the  peculiar 
characters  of  that  group.  From  the  second  period  on  we 
have  the  recurrence  of  periods  of  17,  or  rather  double  per- 
iods of  7  with  an  intermediate  3.  This  gives  in  each  col- 
umn or  group  two  series  presenting  analogies,  and  yet 
striking  differences.  This  was  called  by  Meyer  ' '  double 
periodicity,"  by  Mendeleeff  "matched  and  unmatched 
series. ' '  A  close  study  will  show  that  both  series  show 
points  in  common  with  the  elements  of  the  second  period. 
Thus  both  potassium  and  copper  resemble  sodium  ;  cal- 
cium and  zinc  resemble  magnesium,  and  much  more  than 
they  resemble  one  another,  etc.  This  is  only  slightly 
indicated  in  Mendeleeff 's  arrangement.  One  series  is 
placed  beside  the  other  and  a  little  lower.  An  examina- 
tion of  the  latter  two  tables  will  reveal  that  both  give  a 
better  comprehension  of  the  facts  mentioned  above  than 
does  the  table  of  Mendeleeff.  Thus,  taking  the  last  one, 
the  group  elements  are  those  of  the  first  period.  The 


1 84  A  STUDY  OF  THE  ATOMS. 

type  elements  are  those  of  the  second,  and  from  them 
branch  the  right  and  left  series.  On  the  positive  side  of 
the  table  the  left-hand  series  show  greater  analogies  to  the 
type.  On  the  negative  side  the  elements  of  the  right-hand 
series  resemble  the  type  more  closely. 

The  periodicity  of  the  elements  may 

be  Seen  from  the  following  examples  : 
Taking  the  members  of  the  second 
horizontal  series  the  oxides  and  hydroxides  progress  reg- 
ularly from  left  to  right :  NaA  Mg2O2  (MgO),  A12O$, 

si  A  (Sio,),  PA,  SA  (so,),  CIA- 

NaOH,  Mg(OH)2,  A1(OH)3,  Si(OH)4,  PO(OH)S, 
S02(OH)2,  C103(OH). 

So  too  the  chlorides  :  NaCl,  MgCl2,  A1C13,  SiCl4,  PC15, 
S,C14,  IC1S. 

Taking  the  physical  properties  for  the  same  series  : 

Na.        Mg.      Al.        Si.        P.          S.        Cl. 
Spec.  Gray...     0.97      1.75      2.67      2.49      1.84      2.06      1.33 

Atom.  Vol...  24  14         10  ii  16  16  27 

NaO  MgO    A1203  SiO2  P2O5  SO3  C12O7 
Spec.  Gray...     2.8        3.7       4.0        2.6         2.7        1.9        ? 

Atom.  Vol  ...  22  22         25  45  55  82  ? 

Again,  if  we  take  the  first  two  vertical  groups  we  find 
the  atomic  weights,  specific  gravities,  atomic  volumes  and 
specific  heats  show  a  peculiar  dependence  upon  or  rela- 
tionship to  one  another. 

Li.  Be. 

7.02  9.0 

0.59  1.64 

11.9  7.0 

0.9408  0.4702 

Na.  Mg. 

23-05  24.3 

o.97  1.743 

23-5  13-95 

0.2934  0.245 


THB   PERIODIC   OR   NATURAL   SYSTEM. 


K. 

Cu. 

Ca. 

Zn. 

39-H 

63.6 

40. 

65-3 

0.865 

8.8 

1.58 

7-2 

44-88 

7-o 

25.6 

9.04 

0.166 

0.095 

0.1686 

0.095 

Rb. 

Ag. 

Sr. 

Cd. 

85.5 

107.9 

87.6 

112. 

1.52 

10.5 

2.54 

8.65 

56.4 

10.3 

33-9 

12.69 

0.073 

0.055 

0.074 

0.056 

Cs. 

Au. 

Ba. 

Hg. 

132.9 

197.3 

137.43 

200. 

1.88 

19.5 

3-75 

14.19 

70.68 

10.24 

36.54 

14.12 

0.047 

0-035 

0.046 

0.031 

Difficulties  of 
the  System. 


The  table  of  the  Periodic  System  is  in- 
complete. Between  60  and  70  elements 
have  found  places.  More  than  12 
others  are  known  and  new  ones  are  occasionally  announced. 
The  question  naturally  arises  whether  these  will  all  fit 
into  the  vacant  places  in  the  table.  More  than  30  ele- 
ments can  be  placed  without  changing  the  arrangement. 
The  belief  is  justified  that  if  the  60  odd  known  elements 
all  drop  into  their  proper  order  without  difficulty,  the  re- 
mainder will  also  do  so.  It  is  mere  idle  juggling,  however, 
to  attempt  to  locate  substances  whose  elementary  nature 
is  not  positively  proved,  the  atomic  weights  imperfectly 
known,  and  the  properties  and  compounds  practically  un- 
studied. As  the  table  is  based  upon  all  of  the  properties, 
and  not  the  atomic  weights  alone,  it  has  been  urged  with 
reason,  that  only  elements  that  form  compounds  and  obey 
the  laws  of  affinity  and  valence  can  possibly  enter  into  the 
table.  It  is  readily  conceivable  that  other  elements  may 
exist.  Their  masses  would  have  weight  but  they  might  lack 
that  property  which  would  cause  them  to  attract  and  be 


1 86  A  STUDY  OF  THE  ATOMS. 

attracted  by  other  bodies  so  as  to  unite  with  them  in  com- 
pounds. Of  course,  should  they  not  be  able  to  form  com- 
pounds they  could  exhibit  no  valence.  It  would  seem 
altogether  incongruous  to  consider  and  classify  such  bodies 
among  the  active  chemical  elements,  depending  for  their 
classification  solely  upon  the  weight  of  their  masses. 
The  fact  that  argon  and  its  companions  have  been  known 
for  several  years  and  all  known  methods  used  in  vain  for 
securing  compounds  of  these  bodies  with  known  chemical 
elements  would  seem  to  indicate  the  possible  existence  of 
a  different  class  of  elements  from  those  commonly  so- 
called.  This  is  a  matter  so  important  for  all  chemical 
theory,  and  of  such  deep  interest  that  it  seems  strange  that 
it  has  not  been  made  the  subject  of  exhaustive  investiga- 
tion instead  of  the  somewhat  desultory  and  unsatisfactory 
attacks  upon  the  problem. 

One  of  the  difficulties  of  the  system  is  the 
proper  placing  of  hydrogen.  This  has 
so  far  met  with  no  adequate  solution. 
With  an  atomic  weight  slightly  greater  than  unity, 
this  element  forms  the  beginning  of  the  ascending 
series.  The  next  known  element  in  the  series  is  lithium 
with  an  atomic  weight  of  7.03,  unless  the  inactive  ele- 
ment helium  (atomic  weight  2.00)  be  considered.  This 
would  give  an  atomic  weight  difference  of  6.022,  which  is 
about  three  times  as  great  as  the  atomic  weight  differences 
for  the  first  periods.  Hydrogen  will  not  fall  in  the  lith- 
ium-fluorine period.  It  is  possible  that  it  may  be  the  first 
element  of  a  period  between  i  and  7,  helium  falling  in  this 
period  and  5  other  unknown  elements.  This  was  sug- 
gested by  Mendeleeff,  and  the  idea  has  been  elaborated 
by  Reynolds  and  others.  Some  of  the  properties  of 
hydrogen  would  ally  it  with  the  elements  of  the  first 


THE   PERIODIC   OR   NATURAL  SYSTEM.  187 

group,  as  valence,  electrochemical  nature  and  the  analo- 
gies of  a  number  of  compounds.  The  divergence  in 
properties  from  the  type  of  the  group  indicate  that  this 
sub-period  would  prove  a  very  remarkable  one. 

The  tables  which  give  hydrogen  as  a  primal  element 
with  lines  radiating  toward  the  elements  of  the  first  period 
assign  it  to  a  position  unjustified  by  its  properties  and  are 
based  upon  or  lead  to  unwarranted  assumptions  as  to  the 
genesis  of  the  elements.  Hydrogen  cannot  be  placed  by 
properties  in  a  position  intermediate  between  lithium  and 
fluorine.  The  only  thing  that  is  certain  is  that  in  the 
ascending  series  hydrogen  must  be  left  out  of  the  count 
if  the  elements  are  to  fall  into  analogous  periods  of  seven. 
The  consideration  of  the  Periodic  Sys- 

the  Elements  tem  bas  given  rise  to  a  num^er  of  SUS" 
gestions  as  to  the  genesis  of  the  ele- 
ments, in  spite  of  the  protest  of  Mendeleeff  who  has 
maintained  that  the  system  had  no  bearing  upon  it  and 
could  not  properly  be  used  as  a  basis  for  any  such  specu- 
lations. It  is  true  that  the  table  of  the  elements  does 
not  give  any  positive  knowledge  as  to  their  origin,  nor 
even  afford  any  very  definite  clue  to  aid  in  an  investiga- 
tion into  this  genesis,  but  it  does  reveal  enough  to  lend 
some  color  to  such  speculations,  and  they  have  an  added 
attraction  from  the  unsolved  problems  connected  with  the 
system. 

Certain  deductions  seem  to  be  warranted,  however.  A 
study  of  the  system  cannot  fail  to  convince  one  that  a  re- 
lationship exists  between  the  elements.  The  idea  held  in 
the  earlier  part  of  the  igth  century  and  voiced  by  Ber- 
zelius,  that  the  elements  are  distinct  and  unrelated 
bodies,  is  no  longer  tenable.  The  kinship  is  in  some  way 
bound  up  with  the  atomic  weight  or  mass,  and  with  the 


1  88  A   STUDY   OF  THE   ATOMS. 

gradation  in  atomic  weight  there  is  to  be  seen  a  gradual 
and  proportional  change  in  the  relationship.  Analogy  in 
properties  here  can  only  mean  analogy  in  nature  and  can- 
not be  a  chance  coincidence,  seeing  that  it  is  systemati- 
cally shown.  The  inference  is  easily  drawn,  though 
of  course  it  is  merely  a  plausible  guess,  that  these  ele- 
ments have  a  common  origin  or  a  common  factor  or  fac- 
tors. Some  of  the  hypotheses  as  to  the  genesis  of  the 
elements  are  given  here  in  order  that  the  nature  of  these 
speculations  may  be  seen.  It  must  be  borne  in  mind, 
that  they  do  not  form  a  part  of  chemical  theory,  and 
that  they  go  far  beyond  the  interpretation  of  observed 
facts. 

In  a  lecture  before  the  British  Asso- 

ciation'    Crookesl    Save  a 


ical  picture  of  the  genesis  of  the  ele- 
ments. He  supposed  first  the  existence  of  a  primal 
substance,  called  by  him  protyle,  in  an  "  ultra-gaseous 
state,  at  a  temperature  inconceivably  hotter  than  anything 
now  existing  ;  so  high  that  the  chemical  atoms  could  not 
yet  have  been  formed,  being  still  far  above  their  dissocia- 
tion point.  *  *  *  But  in  course  of  time,  a  process  akin  to 
cooling,  probably  internal,  reduces  the  temperature  of  the 
cosmic  protyle  to  a  point  at  which  the  first  step  in  granu- 
lation takes  place  ;  matter  as  we  know  it  comes  into  ex- 
istence and  atoms  are  formed.  *  *  *  With  the  birth  of 
atomic  matter  the  various  forms  of  energy  which  require 
matter  to  render  them  evident,  begin  to  act  ;  and  amongst 
others,  that  form  of  energy  which  has  for  one  of  its  fac- 
tors what  we  call  atomic  weight.  *  *  *  The  easiest  formed 
element,  the  one  most  nearly  allied  to  the  protyle  in 
simplicity,  is  first  born.  Hydrogen,  or  shall  we  say  helium, 

1  Chem.  News,  54,  117. 


THE   PERIODIC  OR   NATURAL  SYSTEM.  189 

of  all  the  known  elements  the  one  of  simplest  structure 
and  lowest  atomic  weight,  is  the  first  to  come  into  being." 
Thus,  by  cooling,  the  various  elements  are  formed,  slow 
cooling  giving  distinctly  different  elements  with  notable 
difference  in  atomic  weights,  and  more  rapid  cooling  giv- 
ing such  analogous  groups  as  iron,  cobalt  and  nickel  with 
slight  differences.  Crookes  made  use  of  the  diagram  of 
the  periodic  system,  as  devised  by  Spring  and  Reynolds, 
in  which  the  elements  lie  on  an  irregular  curve  described 
by  a  constantly  lengthening  pendulum  on  which  are  laid 
off  the  atomic  weights.  The  vertical  line  may  represent 
a  sinking  temperature,  and  the  oscillations  the  effect  of 
electricity  or  chemical  energy. 

It  is  scarcely  necessary  to  criticize  so  fanciful  a  concep- 
tion. Little  claim  to  originality  can  be  made  for  it.  Ten 
years  before,  Lockyer1  had  announced  a  ' '  working  hy- 
pothesis "  as  to  the  genesis  of  the  periodic  system.  This 
was  based  upon  the  examination  of  stellar  spectra.  The 
hotter  a  star  the  more  simple  its  spectrum  seems  to  be, 
the  chief  lines  being  those  of  hydrogen.  The  cooler  ones 
contain  a  much  larger  number  of  metallic  elements  and 
the  coolest  furnish  band  spectra  characteristic  of  com- 
pounds of  elements.  This  furnishes  the  framework  of  his 
hypothesis. 

The  hypothesis  of  Crookes  is  cited  here  more  for  pur- 
poses of  comparison  with  the  early  Greek  philosophy.  It 
is  an  example  of  the  confused,  imperfectly  wrought  out 
thinking  of  the  present  day,  which  would  not  have  been 
tolerated  by  the  Greek  schools.  The  two  great  theories 
are  recklessly  mingled.  Continuous  matter,  vaguely 
called  ultra-gaseous,  becomes  * '  granulated  ' '  or  atomic. 
It  is  not  matter  such  as  we  know  in  the  visible  universe, 

1  Nature,  19,  197. 


1  90  A  'STUDY  OF  THE  ATOMS. 

yet  what  it  is  seems  a  necessary  question.  It  "con- 
tains within  itself  the  potentiality  of  atomic  weight  "  and 
all  forms  of  energy.  Force  is  not  born,  matter  is  not 
created.  There  is  only  a  vague  something,  and  yet  heat 
and  cooling  and  electricity  and  chemical  energy  are  all 
taking  part  in  the  process. 

The  evolution  theories  of  Wendt,1  Pry  or1 

TU°  U-  I0n  and  others  have  no  foundation  in  observed 
Theories. 

facts,  and  no  attempts  to  adduce  facts  in 

support  of  them  have  been  made.  They  are  in  the  main 
modified  arrangements  of  the  Periodic  System  in  which 
the  authors  see  either  an  undefined  and  unexplained 
evolution  of  the  elements  from  those  of  lowest  atomic 
weight,  or  a  graded  condensation  of  these  lower  elements 
resulting  in  the  formation  of  those  of  greater  atomic 
mass.  It  is  unnecessary  to  describe  these  speculations  in 
detail. 

The  arguments  in  behalf  of 


. 

elements  may  well  be  given 

here.  The  reasoning  is  mainly  from  analogy,  which 
must  often  be  made  use  of  in  science,  and  yet  it  is  not 
always  safe.  When  these  arguments  are  duly  weighed, 
however,  they  cause  a  wavering  in  the  old  faith  as  to  the 
simplicity  of  the  elemental  atoms.  Thorpe3  writes  of  the 
*  *  old  metaphysical  quibble  concerning  the  divisibility  or 
indivisibility  of  the  atom.  '  '  To  Graham  the  atom  meant 
something  which  is  not  divided,  not  something  which 
cannot  be  divided.  The  original  indivisible  atom  may  be 
something  far  down  in  the  make-up  of  the  molecule. 

1  "Entwurf  zu  einerbiogenetischen  Grundlage  fur  Chemie  und  Physik," 
1891. 

2  Berlin  Phys.  Ges.,  10,  85. 

3  "  Essays  in  Historical  Chemistry." 


THE  PERIODIC  OR  NATURAL  SYSTEM.  1 91 

Remsen  says  :l  ' '  The  law  governing  the  properties  of  the 
elements  is  known  as  the  periodic  law.  .  .  .  The  so- 
called  elements  are  shown  to  be  related  to  one  another, 
and  it  seems  impossible  in  the  light  of  these  facts  to 
believe  that  they  are  distinct  forms  of  matter.  It  seems 
much  more  probable  that  they  are  in  turn  composed  of 
subtler  elements."  Gladstone  said2  in  his  presidential 
address  before  the  British  association  :  '  *  The  remarkable 
relations  between  the  atomic  weights  of  the  elements  and 
many  peculiarties  of  their  grouping  force  upon  us  the 
conviction  that  they  are  not  separate  bodies  created 
without  reference  to  one  another,  but  that  they  have  been 
originally  fashioned  or  have  been  built  up  from  one 
another  according  to  some  general  plan." 

The  first  argument  as  to  complexity 

Com  ^xlt38  t0         °f  the  atoms  is  dmwn  fr°m  the  mani" 
fest  kinship  shown  by  the  elements 

in  the  Periodic  System.  This  has  been  mentioned  and 
need  not  be  further  elaborated.  A  second  argument  lies 
in  the  analogy  of  such  compounds  as  ammonia  (NHS), 
cyanogen  (CN),  etc.,  to  the  elements.  Thus  the  first 
can  easily  be  classed  with  the  first  group  in  the  Periodic 
System  and  the  second  with  the  seventh  group.  These 
resemble  elements  in  every  respect  except  that  we  can 
decompose  them  and  build  them  up  at  will.  The  pre- 
sumption is  strong  that  the  same  might  be  done  for  the 
elements  proper  if  only  the  suitable  treatment  had  been 
discovered. 

A  further  clue  to  the  nature  of  the  elements  is  afforded 
in  the  remarkable  change  of  properties  in  an  element 
which  can  be  brought  about  by  ordinary  means.  It  is 

1  Pop.  Set.  Monthly,  34,  591. 

2  Chem.  News,  48,  151. 


IQ2  A  STUDY  OF  THE  ATOMS. 

almost  like  the  creation  of  another  element.  Thus  copper 
is  known  in  a  cuprous  and  cupric  condition  and  the  classes 
of  compounds  given  are  as  different  as  if  they  came  from 
different  elements.  This  is  true  of  a  number  of  other 
elements.  This  is  not  adduced  here  as  a  proof  of  the 
complexity  of  the  original  atoms.  It  is  too  obscure  for 
that. 

The  analogy  of  the  elements  to  the  hydrocarbons  has 
often  been  pointed  out  and  has  its  bearing  here.  These 
hydrocarbons  fall  into  groups  or  homologous  series  with 
definite  increments  in  molecular  weight.  A  table  not  un- 
like the  Periodic  System  can  also  be  fashioned  out  of  them. 
They  can  be  looked  upon  as  the  organic  elements  out  of 
which  all  the  organic  world  has  been  built  up,  just  as  the 
ordinary  elements  go  to  form  the  inorganic  side  of  nature. 


CHAPTER  VI. 


Affinity,  The  Atomic  Binding  Force. 


CHAPTER  VI. 
AFFINITY,  THE  ATOMIC  BINDING  FORCE. 

It  was  seen  from  the  very  earliest  times  that  the  hy- 
pothesis of  the  atomic  constitution  of  matter  involved 
also  an  investigation  as  to  the  force  which  brought  about 
the  union  of  the  atoms  and  held  them  in  combination. 
This  was  a  problem  which  the  earliest  philosophers  found 
themselves  incapable  of  solving  because  of  their  general 
ignorance  as  to  the  natural  forces  and  the  paucity  of  their 
observations  and  data.  And  at  the  present  time,  summing 
up  all  of  our  knowledge,  we  can  do  little  more  than  give 
this  force  a  name  and  define  some  of  the  laws  governing 
it,  which  is  about  the  sum  of  our  knowledge  in  the  dis- 
cussion of  all  of  the  forms  of  energy. 

The  oldest  idea  as  to  the  cause  of  the 

4sal*Jj'   -fWS         union  of  two  substances  was  that  they 
of  Affinity. 

must  contain  some  common  principle. 

Thus  Hipprocrates  (460-357  B.  C.)  taught  as  one  of  the 
fundamental  doctrines  that  "  like  would  unite  only  with 
like." 

As  has  already  been  seen,  at  least  two  ideas  were  held 
as  to  the  controlling  power  bringing  about  the  union 
One  was  that  of  the  vovs  or  Intelligence — the  Direct- 
ing Spirit.  The  other  was  that  of  avdyKTf  or  Necessity 
— a  blind  Fate.  These  ideas  appear  to  have  been  drawn 
from  the  varying  beliefs  as  to  the  creation  of  the 
universe. 

The  dictum  of  Hippocrates  gave  rise  to  the  term  at 
present  used,  namely  affinity,  though  this  ancient  belief, 
cherished  for  so  many  centuries  after  his  time,  has  long. 


196  A   STUDY   OF   THE   ATOMS. 

since  been  lost  sight  of.  The  name  affinity  is  said  to  have 
been  introduced  as  follows  r1 

The  term  affinitas  seems  to  have  been  first  used  by  Al- 
bertus  Magnus  to  indicate  the  cause  of  the  union  of  sul- 
phur with  silver  and  the  other  metals.  The  same  expres- 
sion was  made  use  of  by  chemists  following  him  and  in 
very  nearly  the  same  sense  as  at  present.  Glauber,  Boyle, 
Hooke,  Barchufen,  and  others  found  it  useful  to  designate 
the  unknown  combining  force.  Still,  it  was  inferred  that 
some  similarity  must  exist  between  the  combining  sub- 
stances. The  greater  the  affinity,  the  greater  the  chemi- 
cal resemblance.  The  term  Verwandschaft,  or  relation- 
ship, into  which  affinity  was  translated  by  the  German 
chemists,  still  further  emphasized  the  same  idea. 

With  the  1 8th  century  there  came  a  change  in  this  be- 
lief. Boerhaave  sought  to  show  that  affinity  was  also 
evinced  by  dissimilar  bodies  in  their  tendency  to  combine. 
Solution  was  looked  upon  as  an  act  of  affinity,  and  at  first 
it  was  held  that  tin,  silver,  etc. ,  dissolved  in  mercury, 
resins  in  oils,  etc. ,  because  they  were  related,  but  Boer- 
haave maintained  that  the  solution  of  iron  in  nitric  acid 
was  also  an  act  of  affinity  and  that  no  relationship  existed 
between  the  two,  but  that  they  were  essentially  different. 
His  influence,  as  a  teacher  and  the  wide  distribution  of  his 
text-books,  secured  the  introduction  and  general  adoption 
of  his  views  by  chemists.  Yet  physicists  opposed  the  idea 
of  a  new  force.  Geoff roy  tried  to  avoid  this  idea  by  in- 
troducing the  word  rapport.  Thus  two  substances  united 
because  there  existed  a  rapport  between  them.  The  term 
attraction  used  by  Newton  was  adopted  by  Bergman  (as 
Anziehung),  but  was  too  indefinite  and  general  to  dis- 
place affinity,  which  by  that  time  had  been  fully  incorpo- 

1  Kopp  :  "  Gesch.,"  II,  286,  et  seq. 


AFFINITY,    THE   ATOMIC   BINDING   FORCE.  1 97 

rated  into  chemical  literature,  in  spite  of  the  recognition 
that  the  latter  term  was  based  upon  a  mistaken  idea. 

The  knowledge  of  this  force  grew  very 
°f         slowly.     First  it  was  recognized  that  the 

force  varied  in  strength.  While  many 
operations  in  metallurgy  and  in  the  experimental  work  of 
the  earlier  chemists  indicated  this,  and  for  their  success 
were  dependent  upon  it,  there  were  no  theoretical  observa- 
tions prior  to  the  time  of  Glauber.  He  maintained  that 
the  tendency  of  one  body  to  unite  with  another  differs  in 
accordance  with  the  nature  of  the  latter,  and  that  a  sub- 
stance can  bring  about  the  decomposition  of  such  a  union 
when  it  has  a  greater  affinity  for  one  of  the  components 
than  they  have  for  one  another.  Thus  he  says,  that  to 
drive  ammonia  out  of  sal ammoniacum  one  must  use  potash, 
chalk,  or  zinc  oxide,  and  not  just  any  earth;  sand,  clay, 
etc.,  are  without  effect.  The  action  of  the  potash  is  due 
to  the  fact  that  it  bears  a  close  relation  to  all  acids,  is 
very  fond  of  them  and  beloved  by  them,  therefore  it 
clings  to  the  sal  acidum  (hydrochloric  acid)  and  the 
sal  volatile  (ammonia)  is  set  free  and  distilled  off  as  a 
subtle  spirit. 

Thus  there  were  two  approximate  tests  devised  for 
measuring  the  strength  of  affinity  :  the  readiness  of  com- 
bination and  the  displacement  of  one  substance  in  com- 
bination with  another.  Observations  began  to  accumu- 
late. Glauber  and  Stahl  and  others  announced  certain 
' '  affinity  series. ' '  Thus,  for  mercury  the  series  of  affinity 
was  given  as  gold,  silver,  copper  and  iron.  In  1718 
Geoffrey  published  sixteen  tables,  called  by  him  tables  des 
rapports  ;  and  then  followed  a  number  of  tables  by  differ- 
ent chemists,  the  best  and  most  widely  known  being  the 
tables  of  Bergman  in  1775.  The  following  abstract  from 


I98 


A   STUDY   OF  THE   ATOMS. 


the  table  of  Bergman  will  serve  to  show  the  principles 
upon  which  it  was  based : 


Sulphuric  acid. 

Potash. 

Wet  way. 

Dry  way. 

Wet  way. 

Dry  way. 

Baryta 

Phlogiston 

Sulphuric  acid 

Phosphoric  acid 

Potash  and  soda 

Baryta 

Nitric  acid 

Boracic  acid 

Ammonia 

Potash 

Hydrochloric  acid 

Arsenic  acid 

Alumina 

Soda 

Phosphoric  acid 

Sulphuric  acid 

Zinc  oxide 

Lime 

Arsenic  acid 

Nitric  acid 

Iron  oxide 

Magnesia 

Acetic  acid 

Hydrochloric  acid 

L,ead  oxide 

Metallic  oxides 

Boracic  acid 

Acetic  acid 

Copper  oxide 

Ammonia 

Sulphuric  acid 

Mercury  oxide 

Alumina 

Carbonic  acid 

Silver  oxide 

In  the  table  the  order  from  top  to  bottom  gives  the  rela- 
tive displacing  power.  Thus  in  combination  with  sul- 
phuric acid,  where  the  action  takes  place  in  aqueous  solu- 
tions, baryta  is  represented  as  displacing  any  of  the  sub- 
stances placed  below  it,  and  so  with  potash,  ammonia, 
etc.  Where  the  dry  substances  are  subjected  to  heat  the 
order  is  changed  somewhat. 

It  was  recognized  then  that  the  strength  of  affinity 
varied  with  the  temperature.  This  is  the  attractio  electiva 
simplex  of  Bergman.  He  recognized  also  an  attractio 
electiva  duplex.  Macquer  made  use  of  the  term  affinitas 
retiproca,  where  two  bodies  seemed  to  have  nearly  the 
same  strength  of  affinity  for  a  third  substance,  one  re- 
placing the  other  under  slightly  changed  conditions — a 
partial  recognition  of  the  fact  that  affinity  is  dependent 
upon  other  conditions  besides  temperature. 

This  should  have  sufficed  to  show  the  unreliable  char- 
acter of  the  various  tables  offered,  but  chemists  were  slow 
to  give  them  up.  Nor  did  they  value  at  its  true  worth 
the  remarkable  work  of  Berthollet  and  his  conclusion  that 
the  action  of  affinity  was  proportional  to  the  masses  of  the 


AFFINITY,    THE   ATOMIC   BINDING   FORCE.  1 99 

interacting  substances.  This,  properly  understood,  en- 
tirely did  away  with  all  such  tables,  for  a  body  with  lesser 
affinity  could  displace  one  of  greater  provided  it  was 
present  in  a  sufficiently  greater  mass. 

To  sum  up  then,  chemical  force  or  affin- 
Definition  of         it    .    th    name  f     that  form  of  e 

Affinity.  ;.  ,   ,    . 

which  brings  about  chemical  union  and 

holds  substances  in  compounds. 

1.  It  appears  to  act  only  when  these  substances  are 
brought  within  insensible  distances  of  one  another,  or  in 
actual  contact,  as  it  may  be  roughly  expressed. 

2.  It  is  an  elective  force,  acting  the  more  strongly  the 
more  unlike  the  substances  are,  and  showing  very  little 
energy  where  they  closely  resemble  one  another. 

3.  The   strength   of   affinity   varies   readily  with   the 
change  of  certain  conditions,  especially  of  temperature. 

4.  The  relative  affinity  is  dependent  upon  the  masses  of 
the  interacting  substances. 

Whether  the  assumption  of  a  new 

'S"        force  is  necessary>  or  whether  the 
phenomena  of  chemical  change  can 

be  referred  to  one  of  the  other  physical  forces,  has  long 
been  a  disputed  question.  Berzelius,  Le  Sage,  Berthollet 
and  others  have  endeavored  to  do  away  with  the  necessity 
for  the  assumption  of  a  new  force.  The  question  cannot 
yet  be  decided  and  until  the  problem  is  solved  the  assump- 
tion of  a  new  force  is  necessary. 

It  is  well  to  give  some  of  the  views  which  have  been 
expressed  In  Berthollet 's  "Kssai  de  statiquechimique," 
which,  as  I^othar  Meyer  says,  "  stands  in  the  midst  of 
our  immensely  extended  literature  like  a  lost  landmark, 
to  many  perhaps  unknown,  studied  by  the  few,  completed 


2OO  A  STUDY   OF  THE   ATOMS. 

and  perfected  by  none,"  the  author  supposes  that  what 
is  known  as  affinity  is  probably  '  'a  phase  of  the  same  funda- 
mental property  of  matter  as  that  to  which  universal  gravi- 
tation owes  its  origin. "  It  is  evident  that  these  two  phases 
of  force  exhibit  important  differences.  These  he  attributed 
to  the  proximity  of  the  reacting  substances  in  the  case  of 
affinity  and  to  the  influence  of  special  conditions.  The 
complexity  of  chemical  phenomena  and  our  ignorance  of 
them  prevented,  he  thought,  the  application  of  the  prin- 
ciples of  mechanics  to  them.  To  acknowledge  this  would 
remove  chemistry  still  farther  from  the  position  of  an 
exact  science.  To  quote  again  from  Lothar  Meyer  :l 
4 '  If  chemical  phenomena  are  not  to  be  regarded  as  result- 
ing from  the  actions  of  chance,  then  it  must  be  acknowl- 
edged that  they  are  subservient  to  the  general  principles 
of  mechanics,  to  the  laws  of  equilibrium  and  of  motion, 
and  that  '  the  curve  described  by  a  single  atom  is  as  fixed 
as  the  path  of  a  planet,  and  between  the  two  cases  no 
other  difference  exists  save  that  resulting  from  our  ig- 
norance.' 2" 

L,e  Sage3  would  explain  chemical  phenomena  by  the 
movement  of  the  ultimate  particles,  a  conception  which 
has  been  made  use  of  by  physicists  to  explain  all 
attraction  of  matter.  The  efforts  of  Le  Sage  could  not  be 
other  than  crude  at  that  stage  of  the  science  ;  still  the 
kinetic  theory  has  served  to  explain  many  of  the  phe- 
nomena of  molecular  physics,  and  there  is  much  promise 
in  this  direction.  The  development  of  chemical  statics 
and  dynamics  should  be  the  final  aim  of  chemical  research, 
if  the  motion  of  matter  and  the  equilibrium  of  force  is  to 
be  understood. 

1<l  Modern  Theories  of  Chemistry,"  Introduction  (Eng.  Trans.,  Condon, 
1888). 

2  Laplace  :  "  Essai  phil.  sur  les  probabilities,"  ame  Ed.,  Paris,  1816,  p.  6. 
8 1,e  Sage :  "  Essai  de  chim.  mech.,"  1758. 


AFFINITY,    THE   ATOMIC   BINDING    FORCE.  2OI 

Berzelius  offered  as  an  explana- 

tion  of  affinity  the  hyP°thesis  that 
it  was  dependent  upon  electrical 

attraction.  The  rdle  played  by  electrical  attraction  in 
chemical  phenomena  is  certainly  a  most  important  one, 
but  it  is  easy  to  push  this  idea  beyond  the  point  justified  by 
known  facts.  This  was  done  by  Berzelius  in  his  electro- 
chemical theory.  This  theory  seems  to  have  been  first 
a  conception  of  Davy,  a  kind  of  philosophical  vision  of 
the  two  forces,  electrical  and  chemical,  existing  side  by 
side  everywhere  in  nature  and  holding  all  things  in  equilib- 
rium. It  is  not  strange  that  such  an  idea  should  have 
arisen  in  the  mind  of  one  who  had  already  worked  such 
wonders  by  means  of  this  force,  electricity,  whose  study 
and  triumphs  were  just  beginning.  It  was  Berzelius, 
however,  who  really  enunciated  the  theory,  basing  it  upon 
experimental  investigation  and  making  it  the  basis  of  a 
system  of  chemical  classification  which,  modified  accord- 
ing to  increasing  knowledge,  is  still  the  best  that  chemists 
have  to  offer. 

Berzelius  emphasized  the  fact  already  noted  that  chem- 
ical affinity  was  most  strongly  exhibited  between  atoms 
which  were  most  unlike.  The  wide  difference  in  intensity 
of  action  between  different  atoms  was  also  considered, 
some  showing  almost  no  affinity  for  one  another,  and 
others  very  great.  The  explanation  offered  was  that  this 
exhibition  of  affinity  depended  upon  the  electrical  states 
of  the  atoms.  The  basis,  in  fact,  for  these  views  was  two- 
fold. First,  compounds  are  decomposed  by  the  electric 
current,  and  when  thus  decomposed  their  constituents  in- 
variably seek  the  same  respective  poles.  Again,  chemical 
union  can  be  caused  by  the  action  of  electricity,  and 
chemical  action  is  commonly  accompanied  by  electricity. 


202  A   STUDY   OF   THE   ATOMS. 

According  to  the  ideas  of  Berzelius,  each  atom  is  en- 
dowed with  a  certain  quantity  of  electricity,  partly  posi- 
tive and  partly  negative,  which  accumulates  in  particular 
parts  of  the  atoms,  giving  to  each  a  positive  and  a  nega- 
tive pole.  The  atom  as  a  whole,  however,  has  the  char- 
acter of  either  a  positively  or  a  negatively  electrified  body 
because  of  the  preponderance  of  one  or  the  other  kinds  of 
electricity.  When  two  atoms  combine,  their  respective 
charges  of  electricity  are  neutralized.  Of  course  this 
offers  an  explanation  of  the  greater  attraction  between 
unlike  atoms,  as  bodies  similarly  electrified  exert  little  or 
no  attraction  upon  one  another,  while  with  dissimilar 
charges  the  reverse  is  true.  Every  molecule,  then,  was 
built  up  of  two  parts,  one  positively  and  the  other  nega- 
tively electrified,  and  thus  formed  a  dual  structure.  The 
theory  was  known  as  the  Dualistic  Theory.  The  theory 
practically  identifies  chemical  affinity  with  electrical 
polarity. 

It  is  evident  that  an  accurate  measure- 

of  Affinity!"  ment  °f  the  relative  attraction  between 

different  atoms  is  necessary  as  a  pre- 
liminary to  the  study  of  the  force.  The  difference  between 
the  attraction  exerted  between  a  hydrogen  and  a  chlorine 
atom  and  that  exerted  between  hydrogen  and  oxygen,  or 
any  other  atom,  must  be  known,  and  it  must  be  known 
also  whether  this  action  is  dependent  upon  the  nature  of 
the  atom  or  only  upon  the  interacting  masses. 

It  is  a  matter  of  ordinary  observation  that  such  differ- 
ences exist.  Certain  elements  combine  easily  and  form 
stable  compounds,  others  combine  with  difficulty  and  form 
unstable  compounds.  A  certain  rough  gradation  can  be 
observed  also  in  the  affinity  between  an  element  and  the 
members  of  a  group  or  family.  Thus  rough  measure- 


AFFINITY,    THE   ATOMIC    BINDING   FORCE.  203 

ments  can  be  formed  by  careful  observation  of  chemical 
reactions,  but  they  leave  much  to  be  desired  in  the  way 
of  definiteness  and  accuracy. 

In   seeking  to  measure  affinity  by  such 

istur  ing  observation  of  chemical  reactions  it  must 
Influences. 

be  borne  in  mind  that  there  are  disturb- 
ing influences.  Such  an  influence  can  readily  change  the 
order  in  which  the  breaking-up  of  a  union  of  atoms  takes 
place  so  that  it  shall  not  be  determined  by  the  relative 
affinities.  One  of  the  most  common  disturbing  influences 
is  the  change  of  physical  state.  For  instance,  in  a  reac- 
tion between  two  substances,  A  and  B,  a  third  substance, 
C,  may  be  formed  which  is  a  gas  at  the  temperature  of 
the  reaction.  As  each  particle  of  C  is  formed  it  escapes 
from  the  reaction  or,  as  is  said,  from  the  sphere  of  action. 
It  is  manifest  that  a  new  equilibrium  will  be  striven  for, 
the  formation  of  C  will  continue,  resulting  in  a  final 
equilibrium  of  products  possibly  quite  different  from  the 
result  had  C  remained  in  the  sphere  of  action. 

A  concrete  example  is  that  of  sulphuric  acid  acting 
upon  a  chloride.  At  a  slightly  elevated  temperature, 
gaseous  hydrochloric  acid  is  formed  and  this  will  continue 
until  the  sulphuric  acid  has  replaced  all  of  the  hydro- 
chloric, and  the  old  inference  was  that  the  affinity  of  the 
sulphuric  acid  for  the  base  was  stronger  than  that  of  hy- 
drochloric acid.  When  we  compare  these  acids  by  other 
methods,  however,  in  which  they  are  held  under  the  same 
conditions,  the  hydrochloric  acid  is  seen  to  have  the 
stronger  affinity. 

The  chief  disturbing  conditions,  then,  are  those  under 
which  certain  of  the  resulting  products  are  removed  from 
the  sphere  of  action  ;  it  may  be  as  gas  or  as  a  solid  pre- 


204  A  STUDY   OF  THE   ATOMS. 

cipitated  from  solution.  An  instance  of  the  latter  class  may 
be  seen  when  sodium  chloride  is  added  to  lead  acetate. 
As  is  well-known,  lead  chloride  will  be  precipitated,  and 
yet  the  affinity  of  chlorine  for  lead  is  less  than  for  sodium. 
As  to  why  the  reaction  should  begin  at  all  and  even  a 
single  particle  of  lead  chloride  formed  introduces  another 
phase  of  the  study  of  affinity. 

It  is  manifest  then,  that  merely  observation  of  chemical 
reactions  will  lead  to  most  erroneous  ideas  as  to  the  rela- 
tive strength  of  affinities,  besides  failing  to  lead  to  a  defi- 
nite, direct  comparison  with  any  standard. 

Chemical   action  is  attended  by  the 

"Cf  D°!  C^em"  evolution  or  absorption  of  heat.  Let 
ical  Reactions. 

us  consider  the  union   of  hydrogen 

and  chlorine.  It  is  evident  that  a  measure  of  the  affinity 
between  the  atoms  of  these  elements  could  be  arrived  at 
if  the  heat  produced  by  their  combination  could  be  deter- 
mined, and  if  it  could  be  directly  referred  to  the  transfor- 
mation of  potential  energy  of  isolated  atoms  at  rest  into 
kinetic  energy,  the  molecules  produced  being  at  rest. 
But  it  is  not  in  accord  with  our  best  knowledge  to  suppose 
the  atoms  to  be  originally  isolated  nor  at  rest,  and  hence 
unknown  factors  are  introduced  into  the  equation  and  the 
problem  is  a  complex  one.  Of  course,  the  effort  at 
measuring  chemical  affinity  through  the  heat  produced  is 
dependent  upon  Mayer's  Law  of  the  Conservation  of 
Energy,  that  in  the  transformation  of  physical  forces  the 
production  of  one  is  accompanied  by  a  proportional 
expenditure  of  the  other.  The  measurement  of  the 
heat  evolved  in  chemical  reactions  has  led  to  the  develop- 
ment of  the  branch  of  chemistry  known  as  Thermochem- 
istry. 


AFFINITY,    THE  ATOMIC   BINDING  FORCE.  205 

The  first  law  deduced  was  that  of  Lavoisier1  and  La- 
place namely,  that  for  the  decomposition  of  a  compound 
into  its  constituents  the  same  amount  of  heat  is  absorbed 
as  was  evolved  in  its  formation. 

In  1840  Hess2  announced  the  important  principle  that 
in  a  chemical  reaction  the  amount  of  heat  evolved  is  the 
same  whether  the  process  takes  place  in  one  step  or  in 
separate  steps.  This  removed  many  difficulties  which 
lay  in  the  way  of  the  determination  of  this  evolved 
heat.  Thermochemistry  was  further  built  up  by  the 
work  of  Favre  and  Silbermann,  and  especially  by  that  of 
Thomson. 

How  far  do  the  large  number  of 

Deductions  from  observations  gather  help  in  the 

Thermochemistry.  ,    „ 

measurement  and  study  of  affin- 
ity ?  To  examine  this  question  let  us  take  again  the  for- 
mation of  hydrochloric  acid  by  the  union  of  hydrogen 
and  chlorine.  If  the  present  views  are  correct,  the  first 
thing  that  takes  place  is  a  decomposition  of  the  molecules 
of  hydrogen  and  chlorine.  This  means  an  absorption  of 
heat,  and  hence  the  heat  observed  in  the  reaction  is  less 
by  that  amount  than  the  total  amount  evolved.  The  heat 
measured  is  really  the  difference  between  two  quantities 
whose  absolute  values  are  unknown.  This  is  true  of  every 
chemical  reaction,  and  the  heat  evolved  or  absorbed  in 
any  one  reaction  cannot  be  taken  as  a  measure  of  affinity. 
It  is  possible,  however,  to  arrive  at  some  knowledge  of 
relative  affinities  by  the  study  of  analogous  reactions. 
Thus,  in  the  union  of  hydrogen  with  chlorine,  bromine, 
and  iodine  the  heats  of  formation  are  respectively  44.000, 
16.880  and  12.072  calories.  These  are  not  to  be  taken  as 

1  "  Oeuv.  de  lyav.,"  n,  287. 

2  Pogg.  Ann.,  50,  385. 


206  A   STUDY   OF   THE    ATOMS. 

proportional  to  the  affinities  of  chlorine,  bromine  and 
iodine  for  hydrogen,  but  simply  as  varying  in  the  same 
order.  As  Remsen  says  :  ' '  The  difficulties  are  much  in- 
creased in  more  complicated  cases  and  it  will,  therefore,  be 
seen  that  it  is  impossible  to  measure  the  affinity  by  means 
of  the  heat  evolved  in  reactions."1 

It  is  manifest  from  what  has  been  said  that 
Aif  C<it lar  amnity  in  the  strictest  sense,  that  is,  the 

direct  attraction  between  single  isolated 
atoms,  is  never  dealt  with  alone  in  chemical  reactions  and, 
therefore,  does  not  come  within  the  field  of  investigation. 
The  attraction  considered  is  that  of  atoms  in  molecules  and 
is  much  more  complex .  It  is  a  resultant  of  the  affinities  of 
the  atoms  composing  the  molecules.  This  is  the  only  possi- 
ble deduction  from  the  molecular  theory  of  the  day.  All 
measurements  then  must  represent  these  resultants.  An 
instance  of  the  use  made  of  these  measurements  is  seen 
in  the  question  of  the  neutralization  of  acids  and  bases. 

A  study  of  the  heats  of  neutralization 
HeatofNeu-  of  addg  and  bases  has  rendered  it 
tralization. 

possible  to  correct  such  tables  as  those 

given  by  Bergmann  and  others  at  the  close  of  the  i8th  cen- 
tury, giving  a  truer  picture  of  the  affinities  of  the  acids. 
Equivalent  quantities  of  different  acids  are  neutralized  by 
the  same  base  and  equivalent  quantities  of  different  bases 
are  neutralized  by  the  same  acid,  and  the  heats  of  the 
reactions  are  carefully  measured.  The  reactions  are 
studied  in  aqueous  solutions.  The  strengths  of  the  acids 
thus  measured  are  called  by  Thomsen,  who  has  done 
most  of  this  work,  the  acid  avidities.  What  this  strength 
of  avidity  really  is,  of  course,  cannot  be  stated.  It  is  only 
known  as  a  somewhat  vague  property. 

1  Remsen  :  "Theoretical  Chemistry,"  p.  290. 


AFFINITY,    THE   ATOMIC    BINDING   FORCE.  2Oy 

AVIDITIES  OF  THE  ACIDS. l 

i  molecule  nitric  acid i.oo 

I        **        hydrochloric  acid i.oo 

"        hydrobromic     "    0.89 

"         hydriodic          "    0.79 

"         sulphuric  "    0.49 

"         selenic  "    0.45 

' '        trichloracetic    "    0.36 

' '        orthophosphoric  acid o.  25 

"        oxalic  "    0.24 

"        monochloracetic    "    0.09 

"        hydrofluoric  "    0.05 

1 '        tartaric  "    0.05 

"        citric  "    0.05 

11        acetic  "    0.03 

"        boric  "    o.oi 

"        silicic  "    o.oo 

4 '        hydrocyanic  "    o.oo 

If  the  heat  evolved  in  the  reaction  between  acids  and 
bases  is  known,  an  idea  can  be  formed  as  to  what  takes 
place  when  an  acid  acts  upon  a  salt.  As  in  most  of  these 
cases  no  action  is  evident,  it  is  plain  that  light  can  be 
thrown  upon  it  only  by  some  such  means  as  the  measure- 
ment of  the  heat  of  reaction.  It  has  been  shown  that 
some  action  always  takes  place,  and  that  the  base  is  di- 
vided between  the  two  acids,  however  weak  the  free  acid 
may  ordinarily  be  regarded.  Generally,  more  will  go  to 
one  acid  than  to  the  other.  The  division  between  the 
acids  can  be  measured,  and,  since  equivalents  are  used, 
an  idea  of  the  relative  strength  is  obtained.  This  meas- 
urement is  used  in  connection  with  the  table  just  given. 
Probably  the  most  important  lesson  to  be  deduced  from 
this  work  is  that  in  a  solution,  containing  a  base  and  two 
or  more  acids,  the  base  is  always  divided  between  the 
acids  and  does  not  all  go  to  the  one  with  the  strongest 
affinity. 

1  Thomsen  :  "  Thermochemische  Untersuchungen,"  1882,  i,  308. 


2O8  A   STUDY   OF  THE   ATOMS. 

M         A   , .  The  observation  that  a  base  is  divided 

Mass  Action. 

between  the  acids  in  a  solution  and  not 

all  combined  with  any  one,  leads  to  the  influence  of  mass 
in  chemical  reactions.  The  Law  of  Mass  Action,  how- 
ever, was  first  given  by  Berthollet  many  years  before  these 
observations  were  made.  According  to  his  view,  affinity 
was  essentially  the  same  as  gravitation.  The  deduction 
drawn  by  him  from  his  experiments  was  that  "every  sub- 
stance which  tends  to  enter  into  combination  acts  in  pro- 
portion to  its  affinity  and  its  mass."  This  remarkable 
generalization  was  given  to  the  world  at  a  time  when  the 
study  of  the  atom  was  just  beginning  and  when  all  were 
busied  with  the  establishment  of  the  new  laws  of  constant 
and  multiple  proportions,  the  determination  of  atomic 
weights  and  the  amassing  of  other  facts  necessary  for  the 
foundation  of  the  new  chemistry.  It  was  the  culmination 
of  the  old  chemistry,  and  with  the  passing  away  of  that 
and  the  defeat  of  Berthollet 's  contention  against  the  law 
of  constant  proportions,  it  fell  into  undeserved  obliv- 
ion. It  was  many  years  before  it  was  taken  up  again  and 
bore  fruit  in  the  science.  The  criticism  of  Gmelin  in 
1852  will  exemplify  the  estimate  generally  placed  upon 
Berthollet' s  work  by  chemists  during  the  first  half  of  the 
igth  century. 

' '  There  remains  for  Berthollet  the  great  service  of  hav- 
ing tested  with  sharp  insight  the  doctrine  of  affinity  and 
of  having  observed  it  from  a  new  point  of  view,  directing 
attention  to  the  influence  of  cohesion  and  elasticity  upon 
the  exhibited  effects  of  affinity.  But  he  laid  too  little 
weight  upon  the  strength  of  affinity  and  much  too  great 
upon  the  amounts  of  the  substances  entering  into  reaction 
and  upon  the  influence  of  cohesion  and  elasticity.  He 
wrongly  assumed  that  a  substance  which  separated  as  a 


AFFINITY,    THE   ATOMIC   BINDING   FORCE.  2OQ 

solid  was  out  of  the  sphere  of  chemical  action  ;  that  sub- 
stances could  combine  with  one  another  in  all  possible 
proportions,  and  that  a  substance  divided  itself  between 
two  others  in  the  proportion  of  their  chemical  masses.  '  ' 

The  first  support  of  Berthollet's 
v'ews  antical  data  came 


Views.  . 

through  the  observations  of  H. 

Rose1  in  1851.  Rose  pointed  out  that  while  carbon  diox- 
ide and  water  are  reputed  to  have  but  weak  affinities,  yet 
acting  on  a  large  scale  through  the  centuries  they  have 
decomposed  most  stable  and  resisting  compounds  which 
go  to  form  the  earth's  crust.  A  number  of  laboratory 
experiments  showed  the  decomposition  of  various  salts  by 
water.  In  the  case  of  certain  carbonates  and  berates,  it 
was  proved  that,  with  the  increasing  amount  of  water,  the 
acid  was  withdrawn  from  the  salt  in  increasing  amount. 

Margueritte  and  Tissier  undertook  to  confirm  the  de- 
ductions of  Berthollet  in  the  following  manner.  On  dis- 
solving sodium  chloride  in  a  solution  of  potassium  chlo- 
rate more  will  be  taken  into  solution  than  corresponds  to 
the  solubility  of  sodium  chloride  in  pure  water.  From 
this  they  concluded  that  the  more  soluble  potassium  chlo- 
ride and  sodium  chlorate  were  formed  in  the  solution. 
Also,  if  sodium  chloride  is  added  to  a  saturated  solution 
of  potassium  chlorate,  more  of  the  latter  will  go  into  solu- 
tion. Again,  if  barium,  strontium  or  calcium  carbonate 
is  added  to  a  solution  of  ammonium  chloride  which  reacts 
weakly  acid,  the  reaction  becomes  strongly  alkaline,  show- 
ing the  formation  of  ammonium  carbonate,  and  the 
barium,  etc.  ,  are  found  in  the  solution  as  chloride. 

Many  similar  observations  were  added.  Thus,  copper 
sulphate  is  not  reduced  by  glucose,  while  copper  acetate 

1  Pogg.  Ann.,  82,  545. 


210  A  STUDY  OF  THE   ATOMS. 

is.  If  glucose  is  added  to  a  hot  solution  of  copper  sul- 
phate there  is  no  reduction,  but  if  sodium  or  any  other 
acetate  is  added  copper  acetate  is  formed  and  immediate 
reduction  takes  place. 

Again,  we  have  the  decomposition  of  insoluble  salts  by 
soluble.  Barium  sulphate  can  be  changed  into  barium 
carbonate  by  boiling  with  a  solution  of  potassium  car- 
bonate. The  action  is  limited,  however,  and  ceases  long 
before  all  the  potassium  carbonate  has  been  transformed 
into  potassium  sulphate.  The  reaction  can  be  reversed, 
and  barium  carbonate  can  be  rapidly  changed  to  barium 
sulphate  by  boiling  with  a  solution  of  potassium  or  sodium 
sulphate.  In  the  case  of  calcium  and  strontium  the  sul- 
phates are  more  easily  changed  to  carbonate  than  in  bar- 
ium sulphate,  but  the  reverse  reaction  does  not  take  place. 

Rose  was  not  clear  in  his  explanation  of  these  phenom- 
ena, though  he  ascribed  the  complete  decomposition  of 
calcium  and  strontium  sulphate  to  their  greater  solubility 
and  the  partial  decomposition  of  barium  sulphate  to  the 
action  of  the  soluble  sulphate  formed  upon  the  insoluble 
carbonate  formed  at  the  same  time,  thus  forming  again 
insoluble  sulphate. 

Malaguti1  gave  the  following  explanation :  When 
barium  sulphate  is  acted  upon  by  potassium  carbonate,  at 
first  only  barium  carbonate  and  potassium  sulphate  are 
formed.  As  soon  as  these  are  formed,  however,  the 
opposite  reaction  sets  in,  though  slowly  and  with  little 
energy  at  first,  as  but  small  amounts  are  present.  As  the 
first  reaction  goes,  it  becomes  slower  from  the  decrease  in 
the  quantities  of  the  reacting  bodies,  while  the  opposed 
reaction  increased  for  the  opposite  reason.  When  these 
two  reactions  are  equal  in  speed  the  whole  is  in  equilib- 
rium and  apparently  stationary. 

1  Ann.  chim.pharm.,  (3),  51,  328. 


AFFINITY,    THE   ATOMIC   BINDING   FORCE.  211 

A  great  many  such  reactions  were  investigated  by 
Malaguti,  and  certain  generalizations  were  deduced  by 
him,  but  as  they  have  very  little  direct  bearing  upon  the 
influence  of  mass  they  will  not  be  referred  to  further 
in  this  work. 

The  knowledge  of  the  conditions  existing  in  homoge- 
neous solutions  was  further  advanced  by  the  investiga- 
tions of  Gladstone.  The  investigation  of  such  solutions 
presents  many  difficulties  and  uncertainties.  Malaguti1 
had  used  two  salts  with  different  acids  and  bases,  both  of 
which  were  soluble  in  water  but  only  one  soluble  in  alco- 
hol. The  solutions  were  mixed  and  then  an  excess  of 
alcohol  added.  The  precipitate  formed  was  then  ana- 
lyzed— a  method  open  to  serious  objections.  Gladstone2 
used  a  colorimetric  method.  For  instance,  various  ferric 
salts  are  mixed  with  sulphocyanides.  The  blood-red 
ferric  sulphocyanide  is  formed.  He  found  that  all  of  the 
iron  was  never  changed  and  the  amount  of  change  de- 
pended upon  the  nature  of  the  acid  combined  with  the 
iron  and  the  base  combined  with  the  sulphocyanic  acid. 
The  further  addition  of  either  ferric  salt  or  sulphocyanide 
to  a  mixture  of  equal  amounts  of  the  two  increased  the 
amount  of  red  salt  continuously  and  not  step-wise,  as 
would  be  the  case  if  the  interchange  depended  upon  the 
formation  of  new  compounds,  as  had  been  maintained  by 
Bunsen  and  Debus  in  other  cases.  Gladstone  concluded: 

1 .  That  if  two  or  more  binary  compounds  were  mixed 
so  that  all  existing  compounds  are  free  to  act  upon  one 
another,  each  positive  element  enters  into  combination 
with  every  negative  element  and   in  constant,  definite 
proportions. 

2.  These  proportions  are  independent  of  the  manner  in 

1  Ann.  chim.  pharm.,  (3),  37,  198. 
*  Phil.  Mag.,  (4),  9,  535. 


212  A  STUDY   OF  THE   ATOMS. 

which  the  various  elements  were  originally  combined. 
They  are,  further,  not  merely  the  resultants  of  the  differ- 
ent affinities  between  these  elements  but  also  depend  upon 
the  mass  of  each  substance  present. 

3.  A  change  in   the  mass  of  one  of  the  compounds 
brings  about  a  change  in  each  of  the  others,  and  this 
change  progresses  continuously.      A  sudden    step-like 
change  is  possible  only  when  one  substance  combines 
with  another  in  more  than  one  proportion. 

4.  The  equilibrium  arranges  itself  generally  in  a  very 
short  time,  but  often  the  final  condition  is  reached  only 
after  the  lapse  of  hours. 

5.  The  phenomena  are  quite  different  when  precipita- 
tion, volatilization,  crystallization  or  similar  changes  take 
place,  the  equilibrium  continually  changing  with  the  re- 
moval of  any  of  the  compounds. 

Diffusion  and  circular  polarization  have  been  suggested 
as  additional  methods  for  the  examination  of  homogeneous 
solutions.  In  1862  considerable  progress  was  made  by 
the  investigations  of  Berthelot  and  St.  Gilles  upon  the 
formation  of  ethers  by  the  action  of  acids  upon  alcohols. 
These  reactions  were  especially  adpated  to  this  study  be- 
cause they  took  place  slowly  and  because  a  simple  titra- 
tion  revealed  the  progress  of  the  reaction.  Here  we  reob- 
served  typical  cases  of  reciprocal  or  reversible  reactions  in 
which  the  products  formed  by  the  change  in  the  original 
compounds  call  forth  an  opposed  reaction  reforming  the 
original  compounds.  Thus  alcohol  and  acid  form  ether 
and  water,  and  from  the  mixture  of  ether  and  water  acid 
and  alcohol  are  once  more  formed.  The  tendency  is  to  a 
final  state  of  equilibrium.  This  limit  is  nearly  independ- 
ent of  the  temperature.  It  is  dependent  upon  the  relative 
masses  of  the  reacting  substances. 


AFFINITY,    THE   ATOMIC   BINDING   FORCE.  213 

The  union  of  the  elements  in  most 


of  temperature.  Some  can  remain 
in  combination  only  at  very  low  temperatures,  others  are 
decomposed  only  by  very  high  temperatures.  The  opinion 
is  generally  held  that  at  sufficiently  high  temperatures  no 
union  can  take  place  nor  compounds  exist.  At  such  tem- 
peratures chemical  elements,  if  they  exist  as  such  at  all, 
must  be  in  the  atomic  condition.  It  also  seems  to  be  true 
that  at  sufficiently  low  temperatures  there  is  little  exhibi- 
tion of  affinity.  Thus  the  strongest  mineral  acids  fail  to 
react  with  the  strongest  alkaline  hydroxides  when  the  tem- 
perature approaches  100°  above  absolute  zero. 

The  facts  above  noted  confirm  in  the  strong- 
ine  ic  egt  manner  the  application  of  the  kinetic 

theory  by  Williamson.1  He  was  led  to  the 
assumption  that  in  an  aggregation  of  molecules  of  each 
compound  a  continuous  interchange  goes  on.  Thus,  in 
a  vessel  filled  with  hydrogen  chloride  each  atom  of  hydro- 
gen does  not  remain  quietly  in  connection  with  an  atom 
of  chlorine,  with  which  it  first  entered  into  combination, 
but  there  is  a  constant  interchange  of  place  with  other 
atoms.  Suppose  we  mix  hydrochloric  acid  and  copper 
sulphate,  then  the  hydrogen  atom  does  not  merely  inter- 
change with  other  hydrogen  atoms,  but  may  replace  a 
copper  atom.  So,  too,  any  mixture  of  salts  will  reveal, 
when  examined  at  any  time,  the  bases  distributed  among 
the  different  acids.  A  few  years  later  Clausius2  made 
use  of  the  same  supposition  to  explain  the  phenomena  of 
electrolysis.  In  gases  and  liquids  he  assumed  the  mole- 
cules to  be  in  active  motion,  and  that  more  or  less  fre- 
quently phases  arose  in  which  the  molecules  were  partly 

1  Ann  Chem.  (I^iebig),  77,  37. 

2  Pogg.  Ann.,  101,  338. 


214  A   STUDY   OF   THE   ATOMS. 

separated  and  could  exchange  their  components.  He  did 
not  consider,  as  Williamson  did,  that  this  exchange 
affected  all  molecules,  but  it  was  sufficient  if  only  an  oc- 
casional molecule  was  so  decomposed.  With  rise  of  tem- 
perature there  is  a  more  frequent  separation  of  molecules 
and  a  more  rapid  interchange  of  components.  An  impor- 
tant feature  of  the  hypothesis  of  Clausius  is  the  difference 
of  condition  supposed  to  exist  between  the  molecules  at 
any  fixed  temperature. 

This  hypothesis  offers  a  plausible  explanation  of  disso- 
ciation phenomena  as  well  as  those  of  electrolysis.  Disso- 
ciations are  not  sudden  when  such  and  such  a  temperature 
is  reached,  but  are  more  or  less  gradual  phenomena.  The 
hypothesis  also  throws  light  upon  the  state  of  equilibrium 
in  chemical  reactions,  upon  reversible  reactions,  and  upon 
the  influence  of  mass  action.  Equilibrium  in  chemical  reac- 
tions must  be  looked  upon  not  as  static  but  dynamic.  It  is 
no  stationary  equilibrium  of  forces  but  one  of  opposing  pro- 
cesses. Since  this  equilibrium  is  dependent  upon  the  num- 
ber of  molecules  which  bring  about  the  direct  action  and 
of  those  causing  the  reverse  action,  it  must  be  dependent 
upon  the  relative  masses  of  the  different  substances.  All 
reactions  may  be  looked  upon  as  reciprocal.  Only,  if  in 
any  way  one  or  the  other  of  the  products  formed  is  re- 
moved from  the  sphere  of  action  the  reverse  reaction  can- 
not take  place. 

From  an  entirely  different  line  of  reasoning, 
I  neory  Arrhenius  arrived  at  a  similar  theory  of  so- 
lutions to  that  proposed  by  Williamson  and 
Clausius.  In  the  case  of  water  solutions  of  salts,  strong 
acids,  and  bases,  the  relations  observed  between  such  con- 
stants as  the  boiling-points,  freezing-points,  etc.,  and  the 
molecular  weights  do  not  hold  good.  The  calculations 


AFFINITY,    THE   ATOMIC    BINDING   FORCE.  215 

would  show  a  larger  number  of  molecules  than  the  formu- 
las indicate.  It  should  be  stated  that  the  divergence  is 
greater  in  more  dilute  solutions.  In  concentrated  ones 
it  is  often  scarcely  observable.  In  very  dilute  solutions 
of  some  salts  there  are  apparently  2  molecules  present  for 
every  one  added.  Arrhenius  suggests  that  in  these  cases 
the  dissolved  substances  are,  by  the  action  of  the  water, 
separated  partly  or  entirely  into  their  ions.  Thus  in 
sodium  chloride  there  would  be  sodium  ions  and  chlorine 
ions.  Its  name  comes  from  the  Greek  zfy/iz  (I  go)  and  is 
taken  from  the  terminology  of  the  electrolytic  theory 
with  its  kathodes  and  kathions,  anodes  and  anions.  The 
theory  supposes  the  ions  to  be  highly  charged  with  elec- 
tricity. Their  constant  motion  brings  them  into  contact 
with  one  another,  and  thus  combinations  are  being  con- 
stantly formed  and  broken  up.  Under  the  action  of  an 
electric  current  the  ions  positively  charged  seek  the  nega- 
tive pole,  while  those  negatively  charged  seek  the  posi- 
tive pole. 

There  are  many  difficulties  in  the  way  of  the  acceptance 
of  this  theory  and  in  some  respects  it  is  not  a  satisfactory 
solution  of  the  problems.  It  is  still  under  discussion. 
Manifestly  it  is  of  very  limited  application,  whatever  of 
truth  there  may  be  in  it,  since  it  applies  fully  only  to  cer- 
tain bodies  dissolved  in  particular  solvents  and  then  only 
in  very  dilute  solutions. 

The  first  efforts  at  a  mathematical  for- 
mulation  of  the  relation  between  the 
mass  and  the  corresponding  chemical 
action  were  those  of  Guldberg  and  Waage.1  The  funda- 
mental principle  is  that  the  action  is  proportional  to  the 
mass  entering  into  the  reaction.  This  is  virtually  a  re- 

1  "  E)tudes  sur  les  affinites  chimiques,"  1867  ;  J.  prakt.  Chem.,  (2),  19,  69. 


2l6  A   STUDY   OF  THE   ATOMS. 

statement  of  Berthollet's  views.  The  action  of  two  bodies 
upon  one  another  is  then  proportional  to  the  mass  of  each, 
this  mass  being  the  amount  contained  in  a  space  unit.  If 
the  amount  of  one  is  zero  the  action  is  zero.  The  in- 
tensity of  the  interaction  must  be  measured  by  the  prod- 
uct of  the  active  masses.  The  action  is  further  dependent 
upon  the  nature  of  the  bodies,  the  temperature  and  other 
circumstances.  These  influences  were  considered  together 
under  the  name  of  coefficient.  If  /  and  q  represent  the 
active  masses  and  k  this  coefficient,  then 
chemical  force  =  kpq. 
In  the  case  of  reversible  reactions  the  equilibrium  is 


where  &,  p'  and  q'  represent  the  factors  of  the  opposite 
reaction.  If  equivalent  masses  of  the  original  substances 
were  taken  (P,  Q  and  P',  Q')>  they  do  not  remain  in 
chemical  equilibrium  but  an  amount  x  is  transformed  from 
P  and  Q  into  P'  and  Q'.  Thus  P  becomes  P—  x  ;  Q  is 
changed  to  Q—  x  ;  P'  to  P'  -f  x  ;  Q'  into  Q'  -f  x.  If  v  is 
the  total  volume,  then  for  the  active  masses  in  condition 
of  equilibrium  we  have 

P-*          Q-x          P'+*    ,      Q'+* 

*=—>*=  -f->?    ~irq~'  -T~- 

Inserting  these  values  in  the  equation  for  equilibrium, 
the  following  equation  is  gotten  : 


k' 

If  x  is  determined  from  any  special  case,  then-r-  can  be 

calculated  and  thus  can  be  predicted  for  any  chosen 
original  masses  the  size  of  x  and  the  distribution  of  the 
compounds  upon  the  setting  in  of  equilibrium. 

The  important  advance  over  Berthollet  in  this  work  is 


AFFINITY,    THE   ATOMIC    BINDING   FORCE.  2  17 

the  demonstration  that  the  state  of  equilibrium  is  not 
determined  by  the  original  masses  but  by  the  masses 
present  at  the  moment  of  equilibrium.  It  has  been  shown 
that  the  results  obtained  through  the  study  of  the  heats 
of  neutralization  accord  with  the  theory  of  Guldberg  and 
Waage,  and  what  Thomsen  called  the  avidity  of  acids  and 
bases  is  the  same  as  the  coefficient  of  affinity  in  the  equa- 
tion of  Guldberg  and  Waage. 

The  same  authors  expressed  also 

«•  a  f<*mula  *e  relat.ion  bet^n 
the  velocity  of  reaction  and  the 

chemical  equilibrium.  They  gave  the  reaction  velocity 
as  proportional  to  the  active  force 


where  v  =  -=•=•,  the  ratio  of  the  transformed  mass  dx  to  the 
al 

time  dl  ;  T  is  the  force  =  kpq,  and  O  is  a  factor. 
For  the  reciprocal  reaction 

v  =  0(T  —  T)=  0(kpq—k'pf  ?'). 

When    equilibrium  is  reached   v  =  O  and  the  original 
equation  is  restored. 


These  equations  accord  well  with  experimental  results. 

The  law  of  mass  action  has  been  tested 

further  by  Ostwald  by  the  changes  brought 
Methods.  .  J     .    . 

about  in  volumes.     This  is  done  by  obser- 

vations on  the  specific  gravities  of  solutions.  The  ther- 
mochemical  method  is  difficult  and  requires  large  amounts 
of  substance.  The  volume-chemical  method  is  compara- 
tively easy  and  1/50  the  amount  can  be  used.  He  also  made 
use  of  a  third  method,  the  measurement  of  the  coefficient 
of  refraction,  which  could  be  carried  out  with  still  smaller 
amounts.  The  investigation  can  be  extended  to  bodies 


218  A   STUDY   OF   THE   ATOMS. 

insoluble  in  water,  the  solubility  of  such  in  dilute  acids 
giving  a  measure  of  the  coefficients  of  affinity.  Results 
obtained  by  these  methods  coincide  with  those  already 
given. 

The  simplest  method  of  all  is  the  electrical  method. 
This  consists  in  determining  the  conducting  power  of 
solutions  of  various  dilutions.  This  is  the  method  which 
has  been  most  largely  made  use  of. 

It  is  clear  that  in  these  measurements  of 
Conclusions.       .,  n    ,         „.  .  f     ~   . 

the  so-called  coefficients   of  affinity  we 

are  dealing  with  something  quite  vague  and  unknown, 
and  the  bearing  upon  what  has  been  called  affinity  or 
chemical  force  is  far  from  clear.  Still  some  progress  has 
been  made  in  our  knowledge  of  this  force.  The  first  law 
of  the  mechanical  theory  of  heat  referring  to  the  relation 
between  the  forces  is  obeyed,  and  this  is  the  foundation 
of  thermochemistry.  Again  it  would  seem  that  there  is 
some  connection  between  this  attraction  and  the  electrical 
states  of  the  atoms.  Much  stress  has  been  laid  upon 
this,  but  little  is  really  understood  concerning  it.  Thus, 
until  it  is  explained  why  two  bodies  at  rest,  similarly 
charged  with  electricity,  repel  one  another  while  two 
parallel  wires  with  currents  of  electricity  of  the  same 
order  attract  one  another,  we  can  know  little  of  the  effect 
of  electric  charges  in  atoms  in  constant  motion  in  all  di- 
rections and  at  a  high  speed. 

Again,  if  the  kinetic  theory  is  true,  the  attraction  is  of 
such  a  character  as  to  admit  of  freedom  of  motion  among 
the  atoms,  along  with  a  continuous  interchange  of  atoms, 
one  replacing  the  other.  How  this  is  possible  in  complex 
systems  without  a  breaking-down  of  the  system  is  not 
clear,  nor  yet  why  the  interchange  should  be  restricted 
to  certain  atoms  only  and  not  hold  good  for  any  or  all. 


AFFINITY,    THE   ATOMIC   BINDING   FORCE.  219 

Lastly,  the  attraction,  while  elective,  is  exhibited  be- 
tween all  atoms  coming  into  the  sphere  of  action.  Thus 
when  the  compounds  AB  and  CD  are  brought  into  the 
same  sphere  of  action,  even  though  the  affinity  of  A  for 
B  is  many  times  greater  than  A  for  D,  the  attraction  is 
such  that  some  of  the  A  atoms  give  up  B  atoms  and  unite 
with  D,  and  the  larger  the  number  of  D  atoms,  or  of  mole- 
cules of  the  compound  CD,  the  larger  the  number  of  AD 
molecules  formed,  until  most  of  the  original  AB  molecules 
can  be  broken  up.  The  number  of  D  atoms  combined 
with  A  may  be  less  than  o.oi  part  of  the  total  number 
present.  This  is  the  influence  of  mass  and  has  a  most 
important  bearing  upon  the  nature  of  chemical  force.  It 
is  evident  that  we  are  very  far  from  a  satisfactory  under- 
standing of  the  whole  matter. 

In  what  has  preceded,  the  attraction  be- 

Attraction          tween  at°m   and  at°m  has  been  chiefly 
considered.     It  is  not  possible  to  say  how 

closely  this  is  related  to  the  kind  of  attraction  which  ex- 
ists between  molecule  and  molecule,  nor  the  relation  of 
either  to  gravitation.  There  is  a  field  here  for  much  work. 
It  is  of  interest  to  cite  here  a  recent  examination  of  some 
of  the  laws  governing  molecular  attraction.  The  modern 
theory  of  solutions  has  made  it  very  probable  that  the 
total  kinetic  energy  of  a  gaseous  and  a  liquid  molecule  at 
the  same  temperature  are  the  same.  The  internal  laten 
heat  of  vaporization  may,  therefore,  be  taken  as  a  meas- 
ure of  the  work  done  in  increasing  the  distance  apart  of 
the  molecules.  A  study  of  this  relation  by  Mills1  seems 
to  give  some  ground  for  the  belief  that  the  molecular  at- 
traction varies  inversely  as  the  square  of  the  distance  be- 
tween the  molecules,  and  does  not  vary  with  the  temper- 

i  Molecular  Attraction  :/.  Phys.  Chem.,  1902,  p.  209. 


220  A   STUDY   OF  THE   ATOMS. 

ature,  to  this  extent  resembling  the  attraction  of  gravi- 
tation. 


CHAPTER  VII. 


Valence. 


CHAPTER  VII. 
VALENCE. 

Very  closely  connected  with  the  phenomena  of  affinity 
are  those  of  valence.  Here  again  the  question  arises  as  to 
whether  the  assumption  of  a  new  force  is  necessary. 
Affinity  is  sometimes  called  the  qualitative  combining 
\  force,  and  valence  the  quantitative  combining  force.  If 
there  is  no  affinity  between  two  atoms,  then  no  valence 
can  be  exhibited.  Should  an  atom  have  no  affinity  at  all 
for  any  of  the  other  atoms,  then  it  has  no  saturation  ca- 
pacity or  valence.  The  question  of  valence  did  not  arise 
in  chemistry  until  there  had  been  some  development  of 
the  theories  as  to  affinity.  No  necessity  was  felt  for  it 
until  the  number  of  known  compounds  had  been  greatly 
multiplied  and  the  need  for  their  classification  became 
pressing. 

Valence  may  be  defined  as  that  property 

of6  Valence          of  the  atom  which  decides  the  number  of 
atoms  of  some  other  element  with  which 

it  may  combine.  It  does  not  refer  to  the  ease  or  difficulty 
of  combination,  nor  to  the  stability  of  the  compound 
formed,  but  simply  to  the  number  of  atoms  combined 
with  the  atom  under  question. 

If  the  theory  of  atoms  is  accepted  and  the  validity  of 
the  methods  in  use  for  determining  the  number  of  atoms 
in  the  molecule  be  granted,  then  the  following  facts  are 
arrived  at.  There  are  series  of  compounds  whose  compo- 
sition is  represented  by  the  following  formulas  : 
C1H  OH,  NH8  CH4 

BrH  SH2  PH3  SiH4 

IH  SeH2  AsH8 


224  A   STUDY   OF   THE   ATOMS. 

I49O       BeO       B2O8      CO2       N2O5     SO3    Mn2O7    OsO4 
Na2O      MgO     A12O3     SiO,      P2O5      SeO3  RuO4 

K2O        CaO      Fe2O3     PbO2     As2O6     TeO3 

A  glance  at  these  classes  of  compounds  shows  that  cer- 
tain elements  combine  with  hydrogen  in  the  ratios  of  i 
atom  with  i  ;  i  to  2 ;  i  to  3  ;  and  i  to  4.  And  so  in  the 
case  of  the  oxygen  compounds  there  is  a  varying  number 
of  atoms  of  oxygen  taken  into  combination  depending 
upon  the  nature  of  the  element.  The  number  of  atoms  of 
a  standard  element  with  which  a  single  atom  of  an  ele- 
ment will  combine,  has  been  called  the  chemical  value  of 
that  element.  The  power  of  combining  with  a  certain 
number  of  atoms  of  the  standard  is  known  as  the  combin- 
ing capacity,  capacity  of  saturation,  quantitative  combining 
power,  or  the  valence  of  the  atom.  This  has  also  been  de- 
fined as  the  ratio  between  the  equivalent  and  the  atomic 
weight  of  an  element.  The  term  equivalent,  it  will  be 
remembered,  signified  the  number  obtained  by  analysis 
without  the  introduction  of  any  theoretical  considerations. 
It  was  simply  the  combining  number.  Thus,  with  hydro- 
gen as  the  standard,  and  equal  to  i ,  the  equivalent  of 
chlorine  is  3 5. 4  ;  of  bromine,  80  ;  of  iodine,  127  ;  and  these 
numbers  are  also  the  atomic  weights  of  these  elements. 
Therefore,  the  ratio  is  i  and  the  valence  i.  Again,  the 
equivalent  of  oxygen  is  8  ;  of  sulphur  16.  The  atomic 
weights  are  16  and  32  respectively.  Hence  the  valence  of 

oxygen  =  -—  =  2  ;  of  sulphur  =  ^—  =  2. 

o  ID 

It  is  quite  clear  from  what  has  been  said  that  so  long 
as  the  methods  for  determining  atomic  and  molecular 
weights  were  in  question,  and  indeed  the  atomic  theory 
itself  on  trial  with  equivalents  freely  substituted  for 
atomic  weights,  that  no  need  for  the  idea  of  valence  would 


VALENCE.  225 

be  felt.  Indeed  no  clear  conception  of  this  property  could 
arise.  With  a  fuller  knowledge  of  the  molecule  it  became 
evident  that  an  extension  of  the  atomic  theory  was  called 
for.  In  considering  the  union  of  atoms  in  a  molecule, 
two  distinct  conceptions  are  necessary.  First,  that  of  a 
power  bringing  about  the  union  of  the  atoms,  and,  sec- 
ondly, something  which  places  a  definite  limit  to  the  num- 
ber of  atoms  which  can  enter  into  the  union. 

Probably  the  first  conception  of  valence 

the°Idel°n  °  was  in  the  recoSnition  of  the  so-called 
polyatomic  compounds.  This  term  was 
first  used  by  Berzelius1  in  1827,  he  applying  it  to  such  ele- 
ments as  chlorine  or  fluorine  where  he  thought  several 
atoms  of  these  elements  united  with  a  single  atom  of 
another  element.  This  use  of  the  term  does  not  seem  to 
have  received  wide  acceptance.  It  was  applied,  however, 
to  compounds,  and  for  certain  of  these  its  use  became 
general.  Thus  Graham  applied  it  to  the  acids  combining 
with  various  proportions  of  the  bases.  These  were  called 
polybasic  acids.  Odling  and  Williamson  extended  the 
idea  to  the  compounds  which,  according  to  the  theory 
prevailing  at  that  time,  were  built  upon  types.  Thus 
both  the  type  theory  of  Laurent  and  the  substitution  the- 
ory of  Dumas  were  involved  in  the  evolution  of  this  con- 
ception. The  substitution  of  elements  for  one  another 
would  naturally  lead  up  to  the  idea  of  the  relative  value 
of  their  atoms.  This  was  called  by  lyiebig  the  replace- 
ment value. 

As  we  are  dealing  here  with  the  growth  of 
A°'d  4  a  theory,  it  is  important  to  examine  the 

steps  in  detail.  The  earlier  idea  held  by 
Gay-L,ussac,  Gmelin  and  others  as  to  the  formation  of 

1  "Jakr.  d.  Chem.,"  7,  89. 


226  A    STUDY   OF   THE   ATOMS. 

neutral  salts  was  that  in  the  metallic  oxides  i  atom  of 
metal  was  united  with  i  atom  of  oxygen  and  these 
metallic  oxides  united  with  i  atom  of  acid.  Graham's 
work  upon  the  acids  of  phosphorus  showed  that  in  the 
ortho  acids  for  i  equivalent  of  phosphorus  pentoxide 
there  were  3  equivalents  of  what  he  called  "  basic  water" 
which  could  be  substituted  by  equivalent  amounts  of 
metallic  oxides.  In  the  case  of  other  acids,  he  maintained 
that  this  basic  water  was  present  and  the  number  of 
equivalents  of  it  determined  the  number  of  equivalents  of 
metallic  oxides  which  could  enter  into  combination  with 
it.  Therefore,  he  reasoned,  the  saturation  capacity  of 
these  acids  was  dependent  upon  the  basic  water  belong- 
ing to  their  constitutions.  L,iebig  extended  this  to  many 
other  acids  and  distinguished  between  mono-,  di-,  and  tri- 
basic  acids,  and  the  property  was  spoken  of  as  the  ba- 
sicity of  the  atoms. 

The  idea  of  basicity  was  farther  ex- 

The  Work  of  tended  to  the  compound  organic  radi- 
Frankland.  .  1  *  . 

cals  and  played  a  part  in  the  theories 

of  type,  pairing,  etc.,  which  obtained  in  organic  chem- 
istry. In  his  studies  upon  the  organo-metallic  bodies 
Frankland  noticed  that  arsenic  when  united  with  methyl 
changed  its  saturation  capacity.  Arsenic  was  capable  of 
uniting  with  5  atoms  of  oxygen.  The  highest  oxide  of 
cacodyl,  the  arsenic-methyl  compound,  had  only  3  atoms 
of  oxygen.  Similar  observations  on  other  organo-metallic 
bodies  led  him  to  the  following  conclusion  :l  "  When  one 
observes  the  formulas  of  inorganic  compounds,  even  a 
superficial  observer  is  struck  by  their  general  symmetry. 
.  .  .  Without  making  an  hypothesis  as  to  the  cause 
of  this  agreement  in  the  grouping  of  the  atoms,  it  is  clear 

i  Ann.  Chem.  (I^iebig),  85,  368. 


VALENCE.  227 

that  such  a  tendency  exists  and  that  the  affinity  of  the 
atom  of  these  elements  is  always  satisfied  by  the  same 
number  of  atoms  without  any  reference  to  the  chem- 
ical character  of  these  atoms."  All  of  Frankland's 
conclusions  would  not  now  be  accepted,  but  he  deserves 
the  credit  of  first  gathering  the  facts  bearing  upon  it  and 
announcing  this  new  property  of  the  atom.  The  idea  of 
saturation  capacity  was  thus  extended  from  the  radicals 
to  the  elements. 

It  will  be  readily  seen  that  whether  hydro- 
A  Relative  ^eu  unjtes  wifa  i  or  2  chlorine  atoms  is  as 

much  determined  by  the  chlorine  as  the 
hydrogen  atom.  So,  too,  the  fact  that  i  oxygen  atom 
unites  with  2  hydrogen  atoms  is  decided  by  both  the 
oxygen  and  the  hydrogen  atoms.  It  can  not  be  spoken 
of  as  an  inherent  property  of  the  hydrogen  atom,  nor  of 
the  chlorine,  nor  of  the  oxygen,  but  is  rather  a  relative 
property  evinced  only  when  the  different  atoms  come 
within  the  influence  of  one  another  and  is  the  resultant 
of  that  mutual  influence.  All  attraction  is,  of  course, 
mutual  and  relative.  It  is  necessary  that  some  one  ele- 
ment shall  serve  as  a  standard.  It  will  be  seen  that  there 
are  difficulties  in  the  way  of  this.  Still,  hydrogen  is  or- 
dinarily assumed  as  the  standard.  An  atom  which  com- 
bines with  i  atom  of  hydrogen  or  its  equivalent,  is  uni- 
valent ;  with  2  atoms  is  bivalent ;  with  3  is  trivalent ;  with 
4  is  quadrivalent,  etc.  Of  course  where  the  element 
combines  directly  with  hydrogen,  as  chlorine,  sulphur 
and  nitrogen,  there  is  no  difficulty  in  deciding  upon  its 
valence.  Where  it  does  not  combine  with  hydrogen  it 
may  be  compared  with  some  other  element  which  does  so 
combine,  but  here  serious  difficulties  arise.  If  valence 
does  not  mean  the  absolute  value  of  an  atom  but  is  the  result 


228  A  STUDY   OF  THE   ATOMS. 

of  the  mutual  influence  of  different  atoms  and  dependent 
upon  their  nature,  is  it  right  to  assume  that  the  valence 
toward  some  other  atom  can  be  directly  compared  with 
the  standard  ?  Even  a  slight  examination  of  the  com- 
pounds will  show  that  such  a  conclusion  is  not  justified. 
The  valence  of  a  number  of  elements  compared  with  hy- 
drogen differs  widely  from  that  gotten  by  comparison  with 
chlorine  or  oxygen.  Thus  phosphorus  forms  the  com- 
pound PH3.  Its  valence  with  hydrogen  as  a  standard 
would  be  3.  With  chlorine  it  forms  two  compounds, 
PCI,  and  PC15.  Here  its  valence  is  either  3  or  5.  With 
oxygen  it  gives  the  compounds,  P2O3  and  P2O5.  The 
composition  of  water  is  HjO,  and  so  oxygen  would  appear 
to  be  bivalent.  Any  element  which  combines  with 
oxygen  in  the  ratio  of  2  atoms  with  i  may  be  considered 
to  have  the  same  valence  as  hydrogen.  If  the  compound 
with  oxygen  is  in  the  ratio  of  i  atom  with  i ,  the  element 
is  bivalent,  as  oxygen  is.  If  2  atoms  of  oxygen  to  i  of 
the  element,  then  it  is  quadrivalent.  But  most  of  the 
elements  form  several  oxides.  Thus  gold  gives  Au2O 
and  Au2O3  and  we  are  left  in  doubt  as  to  whether  it  is 
univalent  or  trivalent.  Manganese  has  the  following 
oxides:  MnO,  Mn2O3,  Mn3O4,  MnO2  and  Mn2O7,  giving 
thus  wide  range  of  choice.  Taking  extremes,  we  may 
apparently  have  sulphur  bivalent  toward  hydrogen,  quad- 
rivalent toward  chlorine  and  sexivalent  toward  oxygen ; 
iodine  univalent  toward  hydrogen,  trivalent  toward  chlo- 
rine, quinquivalent  toward  fluorine  and  septivalent  toward 
oxygen. 

The  variability  of  valence  has  been  a  dis- 
Valence  puted  point  among  chemists.  It  would 

seem  from  the  standpoint  of  present  knowl- 
edge that  there  is  little  ground  for  doubting  the  variation 


VAI.KNCE.  229 

both  towards  different  elements  and  towards  one  and  the 
same  element.  Remsen1  says  :  '  *  Valence  is  plainly  vari- 
able, if  we  consider  the  composition  of  the  compounds 
which  an  element  forms  as  final  evidence  of  the  valence  of 
that  element.  If  we  consider  valence  as  due  to  something 
residing  in  atoms,  it  is  difficult  to  conceive  of  this  some- 
thing as  being  variable,  any  more  than  we  can  conceive 
of  the  weight  of  atoms  as  variable.  How  can  one  and 
the  same  atom  have  at  one  time  the  power  to  combine  with 
one  univalent  atom  and  at  another  time  three  or  five  times 
that  power  ?  If  it  has  the  power  to  combine  with  five 
univalent  atoms  once,  it  seems  most  natural  to  suppose 
that  it  would  always  have  that  power."  The  opposite 
view  of  the  invariability  of  valence  was  generally  held  at 
first  and  has  been  maintained  very  stubbornly. 

In  developing  the  constitutional  formulas  for  organic 
substances,  Kekule2  assumed  the  valence  of  the  elements 
to  be  a  constant  magnitude.  He  maintained  that  atomic- 
ity was  a  fundamental  characteristic  of  the  atom,  which 
was  just  as  constant  and  unchangeable  as  the  atomic 
weight  itself. 

One  of  the  first  applications  of  the  doctrine  of  valence  was 
to  the  carbon  atom.  Kekul6  assumed  for  this  a  valence 
of  4,  and  the  constitution  of  all  organic  compounds  was 
explained  on  this  hypothesis,  the  dominant  theories  in 
that  field  still  having  this  for  a  basis.  In  later  years  even 
this  stronghold  for  constant  valence  has  received  some 
sharp  attacks.  It  is  not  strange  that  a  constant  valence 
should  have  been  assigned  to  the  other  atoms  and  vigor- 
ous means  used  to  force  the  formulas  for  their  compounds 
into  agreement  with  it. 

1  "Theoretical  Chemistry,"  p.  91. 

2  Compl.  rend.,  58,  510. 


230  A   STUDY  OF  THE   ATOMS. 

Where  the  formulas  did  not  admit  of 

flolecular  forcing  and  the  variation  in  valence  re- 

Combi  nation. 

mamed,    various    special    hypotheses 

were  devised  to  account  for  it.  Thus  it  was  supposed 
that  there  were  two  classes  of  compounds — atomic  and 
molecular.  The  former  were  true  chemical  compounds, 
and  in  them  the  atoms  exhibited  all  of  their  usual  prop- 
erties, including  valence.  In  the  second  class  a  new  force 
was  called  into  play,  acting  between  the  molecules  and 
binding  them  together.  Through  affinity  the  molecules 
are  first  formed,  and  in  them  valence  has  its  part  to  play. 
Then  these  molecules  attract  and  bind  one  another  to- 
gether, and  in  this  atomic  valence  has  no  part.  Thus  we 
have  salts  with  their  water  of  crystallization  in  which  mole- 
cules of  the  salt  are  supposed  to  bind  molecules  of  water. 
Further,  we  have  such  compounds  as  PC15  and  NH4C1. 
This  distinction  was  chiefly  based  upon  the  comparative 
ease  of  dissociation  of  the  so-called  molecular  compounds 
by  means  of  heat.  The  water  of  crystallization  is  more 
easily  dissociated  from  the  salt  than  either  salt  or  water 
can  be.  Phosphorus  pentachloride  readily  decomposes 
into  the  trichloride  and  a  molecule  of  chlorine,  and  ammo- 
nium chloride  becomes  ammonia  and  hydrochloric  acid. 

If  the  investigation  is  restricted  to 

Objections  to  the  f  h  compounds  as  these  it 

Hypothesis. 

might    be    granted    that    enough 

difference  is  shown  in  stability  to  give  some  foundation 
for  the  hypothesis,  but  there  are  a  number  of  other  cases 
in  which  the  assumption  will  not  hold.  Thus  while  the 
explanation  might  suffice  for  PC15  it  will  not  cover  the 
case  of  POC13  which  can  be  volatilized  without  decomposi- 
tion and  has  every  claim  to  be  considered  a  true  chemical 
compound.  Again,  all  the  ammonium  salts  would  have 


VALENCE.  231 

to  be  explained  as  molecular  compounds.  The  analogy 
of  these  bodies  to  the  salts  of  sodium  and  potassium,  which 
are  chemical  compounds,  make  this  manifestly  untenable. 

In  the  case  of  water  of  crystallization  and 
compounds  like  the  double  salts,  the 
common  view  of  an  invariable  valence 
made  some  explanation  like  that  of  molecular  combina- 
tion necessary.  As  a  substitute  for  this  and  indeed  for 
the  valence  idea,  Werner1  offered  the  hypothesis  of  a  co- 
ordination number.  This  coordination  number  was  the 
limiting  number  which  tells  how  many  atoms  can  stand 
in  direct  union  with  another  definite  elementary  atom  in- 
dependent of  the  valence  number.  This  coordination 
number  was  4  or  6  in  the  majority  of  cases.  For  in- 
stance, if  we  take  the  ferric  chloride,  it  appears  that  the 
molecule  FeCl3,  although  saturated,  possesses  still  the 
power  to  unite  with  the  molecule  KC1,  also  saturated,  to 
form  the  compound  FeCl3.3KCl. 

Now,  it  is  assumed  that  in  this  compound,  the  holding 
together  of  the  molecules  is  determined  by  the  fact  that 
the  iron  atom  even  after  the  saturation  of  its  3  bonds  has 
the  power  of  entering  into  direct  union  with  3  more  nega- 
tive radicals.  It  is  also  assumed  that  in  the  above  com- 
pound all  6  chlorine  atoms  are  united  with  the  iron  atom; 
that  in  it  a  radical,  FeCl6,  is  present  whose  existence  finds 
its  explanation  for  the  characteristic  of  iron  to  stand  in 
direct  union  with  6  atoms,  in  the  coordination  number  6. 
The  coordination  number  therefore  brings  to  view  a 
characteristic  of  the  atoms  which  renders  it  possible  to 
refer  the  so-called  molecular  compounds  to  actual  union 
between  definite  atoms. 

Werner  explains,  by  the  consideration  of  space  relations, 

1  Ztschr.  anorg.  Chem.,  3,  267. 


232  A   STUDY   OF   THE   ATOMS. 

why  the  number  6  plays  so  important  a  r61e.  If  one  as- 
sumes the  atom  to  be  a  material  point,  and  that  the  others 
directly  combined  with  it  are  found  upon  a  sphere  de- 
scribed about  the  chief  atom,  then,  since  the  space  is 
limited,  only  a  definite  number  of  atoms  can  find  place 
there  so  as  to  preserve  a  stable  equilibrium.  This  limit- 
ing number  is  the  coordination  number.  If  it  is  6,  then 
the  simplest  assumption  is  of  an  octahedral  arrangement. 
For  4,  the  symmetrical  position  is  that  of  a  plane.  This 
coordination  number,  therefore,  is  connected  with  the 
space  which  the  atoms  occupy  and  has  nothing  to  do  with 
the  valence,  which  remains  unchanged.  In  the  compound 
mentioned  above,  the  iron  atom  remains  trivalent  and  the 
6  chlorine  atoms  together  sexivalent.  It  is  not  necessary 
to  follow  the  hypothesis  as  further  elaborated  by  Werner. 

The  following  ingenious  proof  is 
Nitrogen  Both  Tri-  d  d  b  R  ,  h  h 

valent  and  Quin- 

quivalent.  nitrogen  may  be  both  trivalent 

and  quinquivalent,  or  that  ammo- 
nium chloride  and  analogous  compounds  of  nitrogen  are 
true  atomic  and  chemical  compounds.  If  NH4C1  is  a 
molecular  compound,  then,  as  was  explained  above,  two 
forces  are  concerned  in  the  formation  of  its  molecule. 

1.  A  force  holding  together  the  nitrogen  atom  and  3 
hydrogen   atoms   forming  the  molecule   NH3,    and  the 
hydrogen  atom  and  chlorine  atom  forming  the  molecule 
HC1. 

2.  A  force  holding  together  the  molecule  NH3  and  the 
molecule  HC1. 

If  these  two  forces  are  distinct  in  character  the  result- 
ing molecule  may  be  represented  by  the  formula 
NH3  -f  HC1.  Suppose  now  we  add  together  two  other 

1  Loc.  cit.,  p.  95. 


VALENCE.  233 

molecules  such  that,   taken   together,   their  constituent 
atoms  are  the  same  in  number  and  quantity  as  those  con- 
tained in  the  compound  NH3  -f  HC1.     Then  the  resulting 
compound  ought  not  to  be  identical  with  that  obtained  in 
the   former  case.     If  these   new  molecules  are,   for  in- 
stance,   NH2C1   and   H2,    then    the    compound    will   be 
NH2C1  +  H2   and    this    should    not    be    identical    with 
NH8  -f  HC1  although  its  composition  is  exactly  the  same. 
This  method  of  investigation  has  been  applied  to  the 
study   of  the  problem  under   consideration,  not   indeed 
with  the  molecules  employed  in  the  above  explanation 
but  with  molecules  analogous  to  them.     Instead  of  NH8 
the  analogous  compound  N(CH3)3  was  taken  and  this  was 
united  with   C2H5I.     Thus   a   compound  was  obtained 
which,  if  it  be  molecular,  should  be  represented  by  the 
formula     N(CH3)3  +  C2H5I.      Again,      the    compound 
N(CH3)2C2H6  was  taken  and  this  was  united  with  CH3I, 
yielding  a  compound  which,  as  in  the  former  case,  should 
be  represented   by   the   formula    N(CH3)2C2H6-f  CH3L 
Now  these  two  compounds  ought  not  to  be  identical  if 
they  are  molecular  and  not  atomic.     On  comparing  them, 
however,  they  were  found  to  be  in  every  respect  identical. 
From  this  experiment  it  is  concluded  that  the  compounds 
studied  are  atomic  compounds  and  that  in  them  nitrogen 
is  quinquivalent.     The   assumption   of  molecular  com- 
pounds is,  therefore,  unjustifiable  in  most  cases  and  un- 
necessary. 

Abandoning  the   hypothesis  of  two 

Saturated  and  different  combining  forces  to  account 
Unsaturated.  .  .  .  ,  , 

for  the  variation  in  valence,  another 

supposition  has  been  that  an  atom  in  combination  could 
be  either  saturated  or  unsaturated.  When  combined 
with  the  largest  possible  number  of  atoms  it  was  consid- 


234  A   STUDY   OF  THE   ATOMS. 

ered  saturated  ;  with  a  smaller  number  it  was  unsaturated. 
The  test  for  saturation  was  to  see  whether  an  atom  in 
combination  could  unite  with  more  atoms.  Thus,  in 
PC13  phosphorus  is  unsaturated  as  it  has  the  power  of 
taking  on  two  more  chlorine  atoms,  forming  PC15.  In 
CO  carbon  is  unsaturated  as  it  can  combine  with  another 
oxygen  atom,  giving  the  compound  CO2.  This  idea  was 
further  confused  with  that  of  completeness.  In  PC15 
the  phosphorus  atom  was  regarded  as  complete,  in  PC13 
as  incomplete.  It  is  manifest  that  these  names  are  an  in- 
heritance from  the  old  phrase  saturation  capacity  and  that 
they  carry  with  them  ideas  and  analogies  which  have  no 
basis  in  fact.  It  is  safer  and  simpler  to  speak  of  phos- 
phorus in  the  first  case  as  quinquivalent  and  in  the  second 
as  trivalent,  and  the  carbon  as  bivalent  and  quadrivalent. 
No  regularity  is  to  be  observed  as  to  stability.  Sometimes 
the  compound  in  which  the  maximum  valence  is  shown 
is  the  most  stable  and  sometimes  the  one  with  the  lower 
valence.  It  is  not  easily  settled  in  many  cases  as  to 
which  is  the  typical  valence  unless  a  count  of  the  com- 
pounds known  be  accepted  as  the  criterion.  It  is  now 
generally  accepted  that  most  elements  occur  in  compounds 
with  widely  varying  valence,  some  with  three  or  four 
different  valences. 

The  necessity  has  been  felt  for  the  intro- 
duction  of  a  term  to  indicate  the  influence 
exerted  in  holding  one  atom  in  union  with 
another.  Where  the  ignorance  is  so  great  as  to  the  na- 
ture of  this  union,  it  is  natural  that  much  difficulty  should 
be  experienced  in  selecting  a  suitable  term.  Care  must  be 
exercised  to  avoid  conveying  ideas  outside  of  present 
knowledge.  The  term  affinity  has  been  used.  Thus  a 
univalent  element  has  one  affinity,  a  bivalent  has  two,  etc. 


VALENCE.  235 

A  serious  objection  to  this  is  the  confusion  with  the  name 
for  the  combining  force,  which,  as  has  been  shown,  is 
quite  different.  Affinity  determines  the  fact  that  the  atoms 
combine  at  all  and  not  the  number  of  atoms  which  com- 
bine. Links  and  linkage  are  terms  associated  with  spe- 
cific material  union.  Perhaps  the  best  term  is  bonds.  A 
quinquivalent  element  has  5  bonds.  Too  material  a  pic- 
ture of  this  union  should  be  avoided,  and  it  must  always 
be  remembered  that  what  we  are  attempting  to  picture  is 
the  emanation  or  exertion  of  some  immaterial  force  or 
influence  between  two  bodies  in  conjunction  with  one  an- 
other. The  various  names  are  mentioned  here,  because 
they  have  each  been  largely  made  use  of  in  the  literature 
of  the  science.  The  term  valences  has  also  been  used  as 
synonymous  with  bonds. 

It  has  been  asked  whether  all  of  the  bonds 

**E>a  *  A  °f  an  element  are  of  the  same  order  and 

of  Bonds.  .          _  _ 

represent  equal  exertions  of  force.     It  has 

been  supposed,  for  instance,  that  phosphorus  has  three 
stronger  bonds  and  two  weaker,  and  so  too  for  nitrogen, 
because  the  trivalent  compounds  were  more  stable  than 
the  quinquivalent.  This  mode  of  reasoning  manifestly 
will  not  apply  when  the  compound  with  the  maximum 
valence  is  the  most  stable.  Nor  is  it  substantiated  by 
experiment.  It  can  be  shown  that  the  5  bonds  of  nitrogen 
are  all  alike  and  equal  so  far  as  the  most  delicate  methods 
of  observation  go.  In  the  case  of  carbon  this  has  been 
investigated  with  great  care  and  the  same  conclusion 
reached.  There  are  some  grounds  for  thinking,  that  this 
is  not  true  for  all  elements  and,  of  course,  the  possibility 
exists  that  more  delicate  methods  would  reveal  differences 
in  all  the  elements.  It  should  be  added,  however,  that 
Werner  regards  the  valences  as  differing  in  value.  He 


236  A   STUDY   OF  THE   ATOMS. 

speaks  of  the  three  primary  valences  (Hauptvalenzen)  of 
nitrogen  and  the  two  secondary  valences  (  Nebenvalenzen) . 

An  extension  of  the  hypothesis  of  saturation 
and  unsaturation  was  the  hypothesis  of  self- 
saturation,  or,  as  it  was  sometimes  called, 
re-entrant  bonds.  Two  bonds  of  the  same  atom  were  sup- 
posed in  some  way  to  act  upon  each  other,  causing  satu- 
ration. This  gave  what  was  considered  a  complete  com- 
pound having  no  free  bonds.  This  self-saturation  was 
supposed  to  be  easily  overcome,  and  then  other  atoms  held 
in  combination.  The  basis  for  this  lay  very  largely  in 
the  observation  that  the  difference  between  the  number  of 
bonds  was  two.  There  would  seem  then  to  be  two  "latent 
bonds."  While  this  is  usually  the  case,  and  we  have  ele- 
ments which  are  bivalent  and  quadrivalent  and  others 
trivalent,  quinquivalent  and  septivalent,  it  does  not  seem 
to  be  at  all  a  necessity.  Some  elements  are  bivalent  and 
trivalent,  etc.  The  assumption  of  self -saturation  really 
explains  nothing  and  is  unnecessary.  And  so  too,  the 
hypotheses  of  double  and  triple  linkage  add  nothing  of 
value  to  chemical  theory.  There  are  undoubtedly  differ- 
ent conditions  of  union,  and  these  may  be  retained  as 
convenient  names  devoid  of  theoretical  significance. 

It  is  well  to  recognize  that  the 
Radical  Change  ch  .fl  yalence  ig  often  a  most 

in  valence. 

radical  and  far- reaching  one,  in- 
fluencing deeply  what  are  ordinarily  regarded  as  the 
chemical  properties.  Thus  the  change  of  univalent  cop- 
per into  bivalent,  of  univalent  mercury  into  bivalent,  of 
univalent  gold  into  trivalent,  of  bivalent  iron  into  triva- 
lent, etc. ,  gives  distinct  series  of  salts,  cuprous  and  cupric, 
mercurous  and  mercuriq,  aurous  and  auric,  ferrous  and 


VALENCE.  237 

ferric,  etc. ,  differing  almost  as  widely  from  one  another 
as  if  they  were  formed  from  different  elements.  So  pro- 
nounced is  the  difference,  in  fact,  that  Mendeleeff  placed 
certain  of  them,  as  cuprous  and  cupric  copper,  in  different 
groups  in  his  system.  And  this  is  in  one  sense  really  jus- 
tified, for  an  examination  will  show  that  according  to 
chemical  properties  univalent  copper  belongs  to  the  first 
group,  and  bivalent  to  the  short  iron  group,  or  to  the 
second  group,  etc.  A  large  number  of  elements,  positive 
and  negative,  form  these  different  classes  of  salts  on 
changing  valence.  This  property  has  not  so  far  been  suc- 
cessfully deduced  or  connected  with  the  ordinary  perio- 
dicity of  the  elements,  nor  does  it  seem  wise  to  attempt  to 
arrange  the  elements  exhibiting  it  under  two  or  more 
groups  in  the  system,  as  was  attempted  by  Mendeleeff. 
Such  a  device  would  produce  confusion  and  lead  to  no 
better  understanding  of  the  phenomenon.  It  is  sufficient 
at  present  to  point  out  the  grave  significance  of  the  change. 

The  variability  of  valence  must  be 

accounted  for  in  ™V  theory  as  to 
the  nature  of  valence  and  is  a  most 

important  clue  to  the  solution  of  that  problem.  It  is 
necessary  then  to  look  closely  into  the  agencies  and  con- 
ditions bringing  about  these  changes. 

It  is  a  matter  of  common  observa- 
Changes^  Caused        tion  that  Hght  can  bring  about  phys_ 

ical,  and  the  most  varied  chemical 

transformations.  In  some  cases  it  causes  a  change  ot 
valence  and  this  change  may  be  either  from  a  higher  to 
a  lower  valence  or  vice  versa.  Thus  certain  mercurous 
compounds  can  be  changed  to  mercuric,  that  is,  univalent 
to  bivalent. 


238  A   STUDY   OF   THE   ATOMS. 

Hg20  =  HgO  +  Hg. 

An  alcoholic  solution  of  ferric  chloride  is  changed  by 
light  to  ferrous  chloride,  a  change  of  trivalent  to  bivalent. 

2FeCl3  +  C2H6O  38  2FeCl2  -f  C2H4O  +  2HC1. 
Ferric  oxalate  under  the  influence  of  light  gives  off  car- 
bon dioxide  and  becomes  ferrous  oxalate. 

Fe2(C204)2  =  2Fe(C,OJ  +  2CO2. 

An  alcoholic  solution  of  cupric  chloride  becomes  cuprous 
chloride.  Mercuric  chloride  in  aqueous  solution  is  slowly 
changed  to  mercurous  chloride  when  exposed  to  light. 

2HgCl2  +  H2O  =  2HgCl  -f  2HC1  H-  O. 
Auric  chloride  in  contact  with  organic  substance  when 
exposed  to  light  is  changed  first  to  aurous  chloride  and 
then  to  metallic  gold. 

Now  the  chemical  action  of  light  is  generally  attributed 
to  the  vibrations  set  up  among  the  molecules.  Rays 
having  the  shortest  wave-lengths  and  the  greatest  fre- 
quency are  most  active  in  this  respect,  though  all  the 
rays  of  the  spectrum  have  been  shown  to  exert  some 
action. 

Variations  in  valence  are  very  fre- 

quently  CaUSed  by  heat     These  are 
commonly  from  a  higher  to  a  lower 

valence  and  are  classed  as  dissociation  phenomena.  Thus 
cupric  chloride  becomes  cuprous  chloride. 


Mercurous  chloride  is  temporarily  changed  into  mercuric 
chloride,  the  mercurous  reforming  on  cooling. 

2HgCl  -  Hg  -f  HgCl2. 
Phosphorus  pentachloride  becomes  the  trichloride. 

PC15  =  PC13  4-  C12. 
Arsenic  pentoxide  becomes  trioxide. 


VALENCE.  239 

An  interesting  series  of  changes  are  those  in  the  sul- 
phur chlorides.  Thus  sulphur  tetrachloride  (SC14)  be- 
comes sulphur  bichloride  (SC12)  if  warmed  above  — 22°, 
and  this  becomes  sulphur  monochloride  (S2C12)  if  heated 
above  64°.  This  last  can  be  boiled  without  change. 
These  instances  might  be  multiplied,  but  it  is  not  neces- 
sary. 

The  most  plausible  explanation  offered  as  to  the  effect 
of  heat  in  bringing  about  chemical  change  is  a  change  in 
the  velocity  of  vibration.  Thus,  L,.  Meyer1  says,  "If, 
therefore,  the  atoms  composing  a  molecule  are  in  motion, 
it  is  evident  that  they,  by  continued  accelerated  move- 
ment, may,  at  last,  be  so  far  removed  from  one  another 
as  to  escape  entirely  the  force  of  affinity,  active  only 
within  narrow  limits,  and  be  unable  to  return  within  the 
sphere  of  its  action." 

Changes  of  valence  due  to  electricity 

KhaJ?i*e?  9*"sed  are  not  unusual.  Thus  we  have  the 
by  Electricity.  ... 

production  of  carbon  monoxide  from 

carbon  dioxide  by  the  passage  of  the  electric  spark. 

CO2  =  CO  +  O. 

In  general  such  changes  may  be  attributed  to  chemical 
action  induced  by  the  electricity  serving  as  the  direct 
agent.  The  change  may  be  the  result  of  changed  vibra- 
tion or  to  changes  of  electrical  state. 

The  most  usual  method  of  bring- 

in*abottt  achangeofvatence  isby 
chemical  action.  These  changes 
are  frequently  very  complex.  As  meager  as  our  present 
knowledge  is,  it  does  not  seem  to  be  a  very  hopeful  task 
to  enter  the  maze  of  changes  of  valence  through  chemical 
reactions  with  a  view  to  clearing  up  the  ideas  as  to  the 

i  "  Modern  Theories  of  Chemistry,"  I^ondon,  1888,  p.  379. 


240  A   STUDY   OF   THE   ATOMS. 

nature  of  valence.  A  few  examples  may  be  taken.  When 
manganese  in  a  septivalent  state  and  iron  in  a  bivalent 
state  come  into  the  same  sphere  of  action,  the  manganese 
is  changed  from  its  highest  valency  to  its  lowest  and  the 
iron  from  its  lowest  to  a  higher. 
2KMnO4  +  icFeSO,  +  8H2SO4  = 

5Fe2(SOJ3  +  K2S04  +  2MnS04  +  8H,O. 
The  simplest  explanation  would  seem  to  be  that  these 
vibrating  systems  are  unstable  in  the  presence  of  one  an- 
other.    Bring  together  the  three  systems,  FeCl3  H2SOa 
and  H2O, 

2FeCl3  +  H2SO2  +  H2O  =  H2SO4  +  2HC1  +  2FeClr 
Whether  we  are  dealing  here  with  a  play  of  affinity, 
which  causes  the  tumbling  down  of  certain  molecules  and 
building  up  of  others,  or  whether  it  is  a  question  of  vibra- 
tory equilibrium  between  these  molecules  cannot  yet  be 
told. 

Various  hypotheses  have  been  sug- 
Sested  to  account  for  this  property  of 
the  atom  known  as  valence.  First  in 
point  of  time  is  the  hypothesis  of  van't  Hoff.1  This  is 
based  upon  the  supposed  form  of  the  atom,  and,  like  most 
of  the  other  hypotheses,  arose  from  a  consideration  of  the 
carbon  atom  and  its  compounds. 

' '  The  simplest  observation  teaches  that  each  change 
from  the  form  of  the  cube  must  lead  to  greater  attractions 
in  certain  directions  since  the  atom  can  be  more  nearly 
approached,  as  it  were,  in  these  spots.  Each  form  of  that 
kind  determines,  therefore,  a  certain  number  of  valences 
or  chief  powers  of  attraction.  Where  the  nature  of  the 
united  atoms  determines  the  attracting  power,  the  num- 
ber also  of  the  valences  exhibited  will  be  dependent  upon 

1  "Ansichten  iiber  die  org.  Chem.,"  I,  3. 


VALENCE.  241 

it,  and  hence  in  comparing  the  compounds  of  a  certain 
element  with  various  others  a  variation  in  valence  will 
often  appear. 

' '  If  an  atom  moves  equally  in  all  directions,  hither  and 
thither,  about  a  definite  position,  a  change  in  the  outer 
form  and,  along  with  that,  in  affinity  and  valence  is  a 
necessary  consequence.  When  one  considers  that  the 
length  of  the  vibration  of  the  atom's  movements  is  de- 
termined by  the  temperature,  the  above  view  leads  to  the 
experimentally  supported  conclusion  that  increase  of 
temperature  lowers  the  number  of  the  valences  and  weak- 
ens the  exhibition  of  affinity ;  in  other  words,  gradually 
reduces  the  interaction  of  the  atoms  to  simple  gravitation 
phenomena.  The  fact  is  that  a  higher  temperature  limit 
exists  beyond  which  chemical  action  is  no  longer  possible. 
And  it  is  also  a  fact  that  on  lowering  the  temperature  the 
chemical  action  becomes  very  complex,  which  is  without 
doubt  to  be  attributed  to  the  overlooked  valences  which 
in  this  way  become  active. 

'  'An  immediate  consequence  of  these  observations  is  that 
a  molecule  made  up  of  atoms  changes  in  the  same  fashion 
as  the  atom  itself,  only  less  sharply,  and  that  the  mole- 
cule has  affinity  and  valence,  which,  indeed,  are  not  in- 
herent and  peculiar  to  it,  but  are  determined  by  its  par- 
ticular composition.  This  will  account  for  the  so-called 
molecular  compounds. ' ' 

This  hypothesis  is  commented  upon  by 
Ostwald's  Qstwald.1  ' '  There  remains  still  a  possibil- 
ity of  explaining  the  actual  difference  in 
the  working  of  valence.  If  we  look  upon  valence  as  a 
question  of  the  characteristics  of  the  atoms,  whose  action 
can  be  modified  by  the  difference  of  condition  of  the  atom, 

i  "  Lehrb.  d.  Allg.  Chemie,"  [i],  I,  830. 


242  A   STUDY   OF  THE   ATOMS. 

especially  the  condition  of  motion,  then  it  is  thinkable 
that  while  the  cause  of  valence  is  unchangeable,  the 
workings  of  this  cause,  even  the  valence  itself,  may  seem 
different  from  time  to  time. 

"An  hypothesis  of  this  kind  has  in  fact  been  put  forth 
by  van't  Hoff.  In  that  he  assumed  that  the  chemical  at- 
traction between  the  atoms  is  a  consequence  of  gravita- 
tion, he  showed  that  if  an  atom  possessed  a  form  varying 
from  that  of  the  cube,  the  intensity  of  the  attraction  upon 
its  surface  must  possess  a  fixed  number  of  maximum  at- 
tractions, which  depends  upon  the  form.  The  maxima 
can  be  of  different  value.  If  the  motion  of  the  atom  due 
to  heat  is  rapid,  then  only  the  greatest  attractions  can 
retain  their  atoms  and  the  valence  shows  itself  to  be 
smaller  by  higher  temperatures  than  by  lower,  which 
accords  with  experience. ' ' 

This  hypothesis  involves  a  consideration  of  the  form  of 
the  atom,  and  the  assumption  that  the  attracting  force  is 
exerted  as  a  maximum  in  certain  directions,  towards  the 
centers  of  the  bounding  faces,  let  us  say.  As  these  faces 
may  be  unequally  distant  from  the  center,  these  maxima 
may  be  unequal.  The  valences  then  or  bonds  will  vary 
except  in  the  case  of  such  figures  as  the  cube  and  the 
sphere.  It  would  appear  that  there  should  be  as  many 
maxima  or  valences  as  there  are  sides,  which  would  give 
a  very  large  number  for  most  geometrical  forms,  which  is 
scarcely  justified  by  experimental  observations  even  at 
low  temperatures. 

Lossen's1  idea  as  to  valence,  deduced  from 

Valence0"         the  consideration  of  the  hypothesis  of  van't 
Hoff   and  Wislicenus  as  to  the  space  rela- 
tions of  the  atom,  seem  to  be  condensed  into  the  single 

1  Ber.  d.  chem.  Ges.,  ao,  3309. 


VALENCE.  243 

sentence  :  "  This  view  leads,  in  my  opinion,  necessarily 
to  the  assumption  that  the  polyvalent  atom  is  not  to  be 
regarded  as  a  material  point,  but  that  rather  parts  of  it 
are  to  be  differentiated  from  which  the  influence  upon 
other  atoms  goes  forth. 

Wislicenus1   expresses   his  ideas  as  to 

Wislicenus  valence  as  follows  :  "  I  consider  it  not 

on  Valence.  . 

impossible  that  the  carbon  atom  is  a 

structure  which  in  its  form,  more  or  less,  perhaps  very 
closely,  resembles  a  regular  tetrahedron,  and  further, 
that  the  causes  of  those  workings  which  exhibit  them- 
selves in  the  so-called  units  of  affinity  (or  bonds)  concen- 
trate themselves  in  the  angles  of  this  tetrahedral  struc- 
ture. These  are  possibly  similar,  and  for  analogous 
reasons,  to  the  electrical  working  of  a  metallic  tetrahedron 
charged  with  electricity.  The  bearers  of  this  energy 
would  finally  be  the  primal  atoms,  just  as  the  chemical 
energy  of  the  compound  radicals  is  undoubtedly  a  re- 
sultant of  the  energy  dwelling  in  the  elementary  atoms." 

The  following  hypothesis  has  been  ad- 

vanced  by  Victor  Meyer  and  Riecke  :* 
and  Riecke.  "We  have  pictured  to  ourselves  the 

following  representation  of  the  consti- 
tution of  the  carbon  atom  upon  the  basis  of  chemical  and 
physical  observations.  We  suppose  this  to  be  surrounded 
by  an  ether  envelope  which,  in  the  case  of  isolated  atoms, 
has  the  spherical  form  as  they  themselves  have.  The 
atom  itself  we  regard  as  the  bearer  of  the  specific  affinities, 
the  surface  of  the  envelope  as  the  seat  of  the  valences. 
Each  valence  we  conceive  as  determined  by  the  presence 
of  two  opposite  electric  poles  which  are  fixed  in  the  ends 

1  Ber.  d.  chem.  Ges.,  21,  581. 

2  Ibid.,  21,  951. 


244  A   STUDY   OF  THE   ATOMS. 

of  a  straight  line,  small  compared  with  the  diameter  of 
the  ether  envelope.  Such  a  system  of  two  electric  poles 
is  designated  as  a  double  pole  or  '  dipole. '  Four  such 
dipoles  would  correspond  to  the  four  valences  of  the  car- 
bon atom.  We  think  of  the  middle  points  of  these  as 
bound  to  the  surface  of  the  ether  envelope  but  easily 
pushed  into  this.  The  dipoles  turn  freely  about  their 
centers."  It  is  scarcely  necessary  to  give  the  further 
assumptions. 

The  hypothesis  of  Knorr1  may  also  be 
Hyp°thesis  given  in  brief.  He  pictures  the  valences, 

or  bonds,  as  determined  by  a  division  of 
the  atoms  into  special  masses,  discrete  and  separate,  which 
he  calls  "valence  bodies"  (Valenzkorper).  Bach  of 
these  valence  bodies  possesses  the  power  of  attracting 
other  valence  bodies  and  of  being  fixed  by  this  attraction. 
The  atomicity  is  determined  by  the  relative  number  of 
the  valence  bodies  present  in  an  atom.  Union  takes 
place  through  the  contact  of  the  valence  bodies.  In  the 
carbon  atom  the  valence  bodies  must  be  of  equal  value 
and  symmetrically  placed. 

Flawitzky2  takes  as  a  basis  for  his  hy- 
of  pothesis  the  suggestion  of  N.  Beketoff 

that  the  cause  of  the  chemical  inter- 
action of  the  elements  lay  in  the  interference  or  coin- 
cidence of  the  motions  of  the  atoms.  The  chief  assump- 
tion is  that  the  atoms  of  each  element  described  closed 
curves  which  lie  in  planes  which  are  parallel  to  one 
another  and  have  a  constant  absolute  position  in  space. 
The  atoms  of  different  elements  move  in  planes  which 
make  definite  constant  angles  with  one  another.  If  one 

1  Ann.  Chem.  Pharm,,  279,  222. 

2  Ztschr.  anorg.  Chent.,  12,  182. 


VALENCE.  245 

considers  the  active  force  of  the  atoms  of  different  ele- 
ments to  be  of  equal  magnitude,  then  the  motion  of  an 
atom  of  one  element  can  be  completely  counteracted  by 
the  motion  of  an  atom  of  another  element  only  when  the 
two  planes  of  motion  are  parallel  to  one  another.  Other- 
wise it  can  happen,  according  to  the  size  of  the  angle  be- 
tween the  planes  of  motion,  that  an  atom  of  i  element 
may  require  2,  3  and  more  atoms  of  another  to  balance 
or  equal  it.  In  such  cases  only  those  components  come 
into  action  which  are  parallel  to  the  plane  of  motion  of 
another  atom.  In  accordance  with  this,  the  valence  of 
an  element  may  be  referred  to  the  difference  in  the 
angles  between  the  path  planes  of  the  different  atoms. 
The  magnitudes  of  these  cycles  must  apparently  follow 
the  law  of  quite  rational  relations  by  which  is  determined 
the  capacity  of  the  atoms  to  combine  in  whole  numbers. 

According  to  Kekule,1  valence  is  purely  a 

Iv  P'fc  1 1  If*  c 

y.  kinetic  question  and  is  determined  by  the 

relative  number  of  impacts  which  I  atom 
receives  from  other  atoms  in  a  unit  of  time.  In  the  same 
time  in  which  the  univalent  atoms  of  a  double-atomed 
molecule  impinge  once,  at  the  same  temperature  the  biva- 
lent atoms  in  a  double-atomed  molecule  come  twice  into 
contact. 

There  have  been  several  attempts2  at  a  mathe- 
y.  er  matical  solution  of  the  problem  of  valence. 

Sedg wick's3  contribution  is  a  mechanical  one 
and,  as  Hinrichsen  remarks,  reminds  one  of  the  view  ex- 
pressed by  fernery  in  the  iyth  century  that  the  combin- 
ing bodies  possess,  respectively,  pores  and  points  and  that 

1  Ann.  Chem.  Pharnt.,  162,  77. 

2  Jaumann  :  Monatsh.  Chem.,  13,  523  ;  Gordon  and  Alexejew  :  Ztschr.  phys. 
Chem.,  35,  610. 

8  Chem.  News,  71,  139. 


246  A   STUDY   OF  THE   ATOMS. 

the  compound  is  formed  by  these  points  entering  the 
pores. 

According  to  Richards,1  the  valence  of 

Richards818  *  an  element  is  Probably  connected  with 
its  compressibility,  since  in  general  the 
greater  the  compressibility,  the  less  is  the  valence.  This 
relationship  is  explained  with  the  help  of  the  hypothesis 
assuming  that  atoms  are  compressible  and  elastic  through- 
out their  whole  substance.  The  carbon  atom,  with  small 
atomic  volume  and  compressibility,  would  naturally 
possess  high  valence,  and  4  larger  atoms  on  combining 
with  it  would  distort  it  into  the  tetrahedron  demanded  by 
the  theory  of  van't  Hoff  and  L,e  Bel.  The  disposition  of 
the  4  added  atoms  on  the  faces,  instead  of  the  points  of 
the  tetrahedron  thus  formed,  would  of  course  make  no 
difference  in  the  geometric  relation.  If  the  4  added  atoms 
were  all  different,  they  would  give  an  asymmetric  distor- 
tion of  the  carbon  atom. 


Hypothesis 
of  Venable. 


According  to  Venable"  there  is  no  neces- 
sity for  the  assumption  of  a  new  force 


nor  any  hypothesis  as  to  the  forms  of  the 
atoms,  the  ether  envelope,  primal  atoms,  valence  bodies, 
etc.  The  question  whether  the  atoms  of  two  elements  will 
unite  is  decided  by  affinity.  The  kinetic  theory  supposes 
a  motion  of  these  atoms  in  the  molecule.  While  one 
speaks  of  union,  there  is  no  actual  contact  to  be  assumed. 
The  individual  atoms  have  their  own  motion  and,  at  the 
same  time,  the  aggregation  of  atoms,  or  molecule,  has  a 
motion  proper  to  it.  In  such  a  molecule  we  can  infer 
from  chemical  reasons  that  there  are  one  or  more  systems, 
depending  upon  the  complexity  of  the  molecule  in  which 

1  Science,  16,  283. 

ZJ.  Am.  Chem.  Soc.,  21,  192,  220, 


VALENCE.  247 

i  atom  is  "united"  with  i  or  2  or  more  atoms.  The 
conditions  of  equilibrium  in  such  a  system  determine 
whether  i  atom  or  2  or  more  atoms  shall  be  ' '  united' ' 
with  a  single  atom.  Two  factors  may  be  considered  in 
this  equilibrium,  the  peculiar  motion  of  each  elementary 
atom  and  the  rate  of  motion  dependent  upon  external 
conditions.  The  latter  is  readily  changed  by  such  agen- 
cies as  heat,  light,  etc.,  and  the  valence  will  vary  with 
the  change  in  this  factor. 

There  is  then  no  distinct  force  of  valence  inherent  in 
the  atoms.  The  atomic  weight  has  little  influence  in 
determining  the  number  of  atoms  needed  to  satisfy  the 
conditions  of  equilibrium  except  that  there  seems  to  be  a 
general  rule  that  with  increase  in  the  atomic  weight  in 
any  one  group  more  stable  equilibrium  is  brought  about 
with  the  smaller  number  of  atoms,  and  in  a  choice  between 
several  the  lesser  valence  is  preferred.  (Compare  nitro- 
gen and  bismuth  ;  sulphur  and  selenium. ) 

A  phosphorus  atom  unites  with  chlorine  atoms  because 
of  a  certain  affinity  between  them.  The  number  of  chlo- 
rine atoms  with  which  it  will  unite  depends  upon  the  pos- 
sibility of  an  equilibrium,  harmonizing  the  respective 
motions.  As  the  temperature  may  impart  a  more  rapid 
molecular  motion,  it  is  evident  that  the  harmony,  or  equi- 
librium, will  depend  more  or  less  upon  the  temperature  and 
that  a  temperature  may  be  reached  at  which  no  2  or 
more  atoms  can  remain  in  equilibrium,  and  hence  no  com- 
pound can  be  formed.  The  phosphorus  atom,  above 
mentioned,  can,  as  we  know,  form  a  stable  molecule  with 
5  atoms  of  chlorine.  On  increasing  the  temperature  this 
becomes  unstable  and  only  3  atoms  can  be  retained. 
Neither  with  4  atoms  nor  with  2  does  there  seem  to  be 
harmony  of  motion. 


248  A  STUDY  OF  THE  ATOMS. 

It  is  manifest  that  with  this  view  there  is  no  necessity 
for  any  assumption  as  to  atomic  and  molecular  combina- 
tion nor  for  Werner's  coordination  number.  As  there  can 
be  an  equilibrium  determined  by  the  motion  of  single 
atoms,  so  there  can  be  an  equilibrium  of  molecules  deter- 
mined by  their  motion.  Thus  the  copper  sulphate  mole- 
cule moves  in  equilibrium  with  5  molecules  of  water,  an 
equilibrium  readily  disturbed  by  heating. 

There  may  be  sets  of  conditions  bringing  about  harmony 
of  motion.  Thus  a  carbon  atom  moves  in  harmony  with 
4  hydrogen  atoms  or  2  oxygen  atoms  or  i  oxygen  atom. 
Valence  is  a  necessary  sequence  of  the  kinetic  theory  ap- 
plied to  atoms.  This  matter  will  be  referred  to  again  at 
the  close  of  the  last  chapter. 


CHAPTER  VIII. 


Molecules    and    the    Constitution  of 
Matter. 


CHAPTER  VIII. 
MOLECULES  AND  THE  CONSTITUTION  OF  MATTER. 

We  come,  at  the  close  of  this  discussion,  back  to  the 
original  question.  How  is  matter  constituted  ?  All  ex- 
perimental research  has  brought  support  to  the  atomic 
theory  of  L,eucippus  in  so  far  as  that  maintains  that 
matter  is  composed  of  separate,  discrete  particles.  These 
are,  in  their  first  analysis,  not  the  atoms  of  L,eucippus 
nor  yet  those  of  Dal  ton,  but  compound  molecules. 
Matter,  to  the  best  of  our  knowledge  and  belief,  is  made 
up  of  molecules  which  are  separable  into  their  component 
atoms  but  which,  within  all  ordinary  experience,  exist  as 
complexes.  In  this  it  should  be  borne  in  mind  that  we 
are  simply  refining  upon  and  elaborating  Dal  ton's  theory, 
which  made  little  distinction  at  first  between  atom  and 
molecule.  Whether  the  term  atom  in  its  ancient  meaning 
of  the  indivisible  particle  can  be  applied  to  the  atoms  of 
Dalton  and  of  modern  chemistry  has  after  all  slight  bear- 
ing on  the  theory,  and,  whatever  interest  the  solution  of 
the  question  may  have  in  itself,  it  can  safely  be  neglected 
so  far  as  the  theory  explaining  the  great  laws  of  chem- 
istry is  concerned.  To  the  mind  of  the  chemist  of  to-day 
the  elementary  atoms  are  almost  surely  complex,  but  he 
cares  little  for  that  in  the  actual  application  of  his 
theories.  The  truth  is,  that  beyond  certain  properties,  such 
as  the  physical  one  of  weight,  meaning  the  attraction  of 
gravitation  upon  it,  little  is  known  concerning  the  ele- 
mentary atom.  Except  in  a  few  cases,  such  as  the  mona- 
tomic  gases  like  mercury,  the  isolated,  individual  atom 
cannot  at  present  be  subjected  to  study,  and  practically 
little  is  known  as  to  its  behavior.  In  all  dealing  with 


252  A   STUDY   OF  THE   ATOMS. 

matter,  it  is  the  molecule  that  comes  under  observation, 
and  experience  has  taught  that  the  atom  is  profoundly 
influenced  in  properties  by  the  presence  of  other  atoms. 
As  has  been  shown,  even  in  simple  elementary  gases  the 
belief  is  justified  that  one  is  dealing  with  two-atomed 
molecules.  In  other  gases  this  is  more  complex,  and  we 
can  reason  that  the  complexity  greatly  increases  as 
we  go  from  gases  to  liquids  and  from  liquids  to  solids 
(although  this  has  been  denied),  finding  molecules  more 
and  more  complex  and  nowhere  the  individual  atom. 

If  the  body  called  hydrogen,  which 

The  Influence  of  -s  known  in  the  moiecular  condition 
Atom  upon  Atom. 

to    possess    certain    properties,    is 

brought  within  the  sphere  of  influence  of  the  body  called 
oxygen,  whose  properties  in  the  molecular  condition  are 
also  known,  and  the  proper  conditions  of  temperature  are 
observed,  union  takes  place,  and  a  molecule  of  water  is 
formed.  This  molecule  of  water  contains  one  atom  of  oxy- 
gen and  two  atoms  of  hydrogen.  The  properties  which 
characterized  the  molecule  of  hydrogen  and  the  molecule 
of  oxygen  have  entirely  disappeared,  and  new  character- 
istics appear,  different  from  the  former  and  in  no  known 
way  connected  with  them  nor  derived  from  them.  This 
change  of  properties  is  observed  in  all  cases  of  chemical 
union,  and  is  taken  as  indicative  of  the  fact  that  chemical 
union  has  taken  place.  On  decomposing  the  compounds 
and  restoring  the  constituents,  the  former  properties  re- 
appear ;  therefore,  they  were  merely  cloaked,  or  rendered 
potential,  with  the  tendency  to  their  restoration  persist- 
ing. A  few  properties,  such  as  the  atomic  weight,  are 
persistent  and  are  not  changed  nor  cloaked  by  the  act  of 
combination. 

This  behavior  must  be  taken  as  indicating  the  profound 


AND  CONSTITUTION  OF  MATTKR.        253 

influence  exerted  by  one  atom  upon  the  other.  No  solu- 
tion of  the  problem  seems  possible  until  the  origin  of  the 
properties  of  an  atom  is  known.  It  is  scarcely  conceiv- 
able that  these  properties  are  in  a  literal  sense  dependent 
upon  the  atomic  weight,  which  is  nothing  more  than  the 
attraction  exerted  by  the  earth  upon  the  individual  atom. 
They  vary  with  the  atomic  weight,  or  reversing  the  view, 
the  atomic  weight  varies  with  them.  As  has  been  stated, 
the  periodic  system  should  not  be  looked  upon  as  an  ar- 
rangement solely  according  to  the  atomic  weights,  but 
according  to  all  of  the  properties. 

The  only  opportunity,  so  far  known,  of  ob- 
ascen  serving  the  atom  in  the  condition  of  freedom 

from  union  with  other  atoms  is  at  the  mo- 
ment of  its  liberation  from  a  molecule  and  before  its  en- 
tering into  combination  in  a  new  molecule.  This  has  been 
called  the  status  nascendi  or  nascent  state  of  the  atom. 
The  interval  is  undoubtedly  exceedingly  brief,  and  affords 
little  opportunity  for  the  observation  of  properties.  The 
only  one  which  has  been  noted  with  any  degree  of  cer- 
tainty is  the  far  greater  chemical  activity  of  the  free  atom. 
One  cannot  be  sure  that  he  is  dealing  with  the  free  atom, 
and  mistakes  have  been  made.  It  appears,  however,  that 
hydrogen  just  liberated  has  a  power  of  breaking  up  ex- 
isting molecules  and  making  new  combinations,  which  is 
not  shown  by  molecular  hydrogen.  Thus  the  stable  ar- 
senic trioxide  is  broken  up  and  compounds  of  arsenic  and 
hydrogen,  and  oxygen  and  hydrogen  formed,  so  too  with 
antimony  trioxide,  nitric  acid,  nitrobenzol  and  many  other 
compounds.  The  same  increased  reactivity  has  been 
noted  in  the  case  of  nascent  oxygen  and  other  elements. 
What  changes  there  are  in  other  properties  when  the  ele- 
ments are  in  the  atomic  state  can  only  be  surmised. 


254  A   STUDY   OF   THE   ATOMS. 

Allot  ism  Something  may  be  inferred  from  what 
has  been  called  the  allotropic  condition 
of  an  element.  A  number  of  the  elements  are  known  to 
exist  in  more  than  one  form.  Thus  there  are  three  well- 
known  forms  of  carbon,  several  of  sulphur,  of  phosphorus, 
of  silver,  of  gold,  etc.  As  only  one  kind  of  atom  can  be 
considered  under  each  heading,  the  only  plausible  explana- 
tion is  that  there  are  molecules  containing  different  num- 
bers of  atoms.  A  familiar  example  is  that  of  oxygen  and 
ozone.  From  a  number  of  different  reasons,  we  can  infer 
that  in  oxygen  the  molecule  has  two  atoms  and  in  ozone 
three.  These  two  forms  of  oxygen  differ  practically  in  all 
properties,  chemical  and  physical,  although  the  constitu- 
tional difference  between  them  is  so  slight.  When  we  come 
to  consider  allotropism  in  the  case  of  most  other  elements, 
no  method  has  been  devised  for  telling  the  number  of 
atoms  in  the  molecules :  we  find  them  very  different,  and 
must  assume  that  the  numbers  of  atoms  differ.  As  these 
atoms  are  all  similar,  the  question  of  their  arrangement 
in  the  molecule  has  not  been  considered  as  a  factor,  though 
unquestionably  it  may  be  one.  The  difficulties  attending 
any  investigation  along  this  line  are  apparent.  The  fact 
remains,  however,  that  the  presence  of  two  or  more  atoms 
of  the  same  kind  also  materially  influences  their  proper- 
ties and  confers  new  properties  upon  the  molecule. 

The   vast   number  of    com- 

^  tichf  exist  rffoi 

any  number  of  examples  of 
atom  influencing  atom,  but  two  or  three  special  cases  may 
be  taken  which  have  a  somewhat  peculiar  interest.  Thus, 
the  properties  of  molecular  carbon  are  fairly  well-known 
in  the  three  different  forms  in  which  it  exists.  When  it 
enters  into  a  molecular  arrangement  with  hydrogen,  these 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        255 

properties  are  profoundly  modified  and  new  ones  appear. 
The  different  molecules  which  can  be  formed  with  vary- 
ing numbers  of  atoms  of  carbon  and  hydrogen  are  ex- 
ceedingly numerous.  In  these,  some  properties  persist, 
such  as  atomic  weight,  and  atomic  heat,  but  other  proper- 
ties are  quite  new.  Thus  we  have  CH4,  C2H6,  C6H6,  C5H8, 
CUH16,  etc.  When  one  other  element  is  introduced, 
namely  oxygen,  we  have  proportions  and  properties 
almost  as  diverse  as  the  organic  nature  surrounding  us. 
In  the  case  of  certain  of  these  hydrocarbons,  we  have 
homologous  series  with  a  regular  increment  of  carbon  and 
hydrogen  atoms.  Thus  there  is  the  methane  series,  CH4, 
C2H6,  C3H8,  C4H10,  etc.,  or  the  ethylene  series,  C2H4,  C3H6, 
C4H8,  C5H10,  etc.  Here  the  changes  of  properties  may  be 
approximately  predicted.  In  other  words,  the  effect  of 
adding  a  molecule,  CH2)  is  understood.  The  same  effect 
is  not  always  produced,  but  it  depends  upon  the  series 
into  which  it  is  introduced  and  the  size  of  the  molecule. 
Again,  the  differences  are  very  noteworthy  when  the 
carbon  atoms  are  under  the  influence  of  all  the  hydrogen 
atoms  with  which  they  can  form  stable  molecules  and 
when  the  hydrogen  atoms  are  less  than  is  demanded  for 
such  perfect  harmony. 

I  some r ism  Again,  different  properties  are  produced 
when  the  same  atoms  are  differently  ar- 
ranged. A  great  many  compounds  are  known  which 
have  the  same  elements,  the  same  ratio  between  them 
and  the  same  molecular  weight,  or  the  same  number  of 
atoms  in  the  molecule.  Thus,  two  substances  are  known 
having  the  formula  C4H10,  three  having  the  formula  C5H12 
and  five  having  the  formula  C6HU.  No  other  plausible 
explanation  is  offered  of  the  existence  of  these  bodies 
other  than  that  they  have  the  atoms  in  the  molecules 


256  A   STUDY   OF  THE   ATOMS. 

differently  arranged.  This  is  called  isomerism.  The  dif- 
ference may  be  comparatively  slight,  as  in  the  case  of  the 
three  mesitylenes  C9H12,  or  very  great,  as  in  the  case  of 
dipropargyl  and  benzol,  C6H6,  thus  indicating  a  greater 
or  lesser  difference  of  arrangement. 

The  arrangement  of  the  atoms  in  a  molecule  then  has 
a  most  important  influence  upon  the  properties  of  the 
molecule.  The  most  plausible  explanation  of  this  is  in 
the  assumption  of  intramolecular  motion  modified  by  the 
changed  atomic  and  molecular  motion  and  the  modifica- 
tions produced  by  the  necessity  for  harmonizing  these 
motions  in  a  system. 

It  is,  furthermore,  a  well-known  fact  of 

Influence  of  chemistry  that  the  relative  position  of 
Position.  J  ,  .  r  , 

the  atoms  has  a  most  important  bearing 

upon  the  properties  of  the  molecule.  The'se  are  in  reality 
somewhat  more  complex  cases  of  isomerism  than  those 
mentioned  in  the  last  paragraph, — substances  containing 
more  than  two  elements  yet  having  the  same  elements, 
the  same  ratio  and  the  same  molecular  weight  and  show- 
ing different  properties.  Thus  we  have  two  bodies  with 
the  formula  C2H6O.  Chemically  and  physically  they  are 
absolutely  unlike.  They  cannot  be  classed  together  at 
all.  Many  reactions,  decompositions  and  syntheses  lead 
to  the  conclusion  that  in  one  case  we  have  two  groups  of 
atoms,  C2H5  and  OH,  united  by  one  of  the  carbon  atoms, 
and  in  the  other  two  groups,  CH3  and  CHS,  united  by  an 
atom  of  oxygen.  The  two  formulas  then  are  written 
C2H5.OH  and  CH3.O.CH3,  or  graphically 

H    H  H  H 

II  II 

(i)   H— C— C— O— H        and       (2)    H— C— O— C— H. 

II  II 

H    H  H  H 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        257 

The  relative  position  of  the  atoms  in  (i),  wherever  it 
occurs,  gives  what  is  known  as  alcoholic  properties.  The 
relation  observed  in  (2)  gives  the  properties  of  ethers. 

Two  bodies  are  known  with  the  formula  CH3CN. 
They  are  very  different  in  properties.  The  conclusion 
reached  is  that  we  have  to  consider  these 

H  H 

(i)  H— C-C— N     and     (2)  H— C— N— C. 


H 

This  is  confirmed  by  many  reactions  and  is  no  mere  assump- 
tion. Thus  the  entire  character  of  the  molecule  is  decided 
by  the  relative  position  of  the  atoms  of  carbon  and  nitro- 
gen as  compared  with  the  radical  group  CH3.  Again  two 
bodies  (i)  C2H5SCN  and  (2)  C2H5NCS  are  known.  They 
differ  in  properties,  and  this  difference  leads  to  an  assump- 
tion of  a  difference  in  arrangement  of  the  atoms  in  the 
molecule.  In  ( i )  one  carbon  atom  of  the  radical  is  united 
with  the  sulphur  atom  or  in  juxtaposition  to  it.  In  (2) 
the  carbon  atom  bears  the  same  relative  position  to  the 
nitrogen  atom. 

Recalling  other  familiar  examples  from  organic  chem- 
istry, we  find  that  the  union  of  an  atom  or  group  of 
atoms,  as  Cl  or  NO2,  to  a  carbon  atom  in  a  hydrocarbon 
which  had  three  atoms  in  union  with  it,  produces  a  dif- 
ferent compound  from  that  formed  by  the  union  with  a 
carbon  atom  which  had  only  two  hydrogen  atoms.  Thus 
the  hydrocarbon 

H    H    H    H    H 

I       I       I       I       I 
H— C— C— C— C— C— H 

I       I      I      I       I 
H    H    H    H    H 

can  form  two  chlorides  and  two  only. 


258  A  STUDY   OF   THE   ATOMS. 

H    H    H    H    H 

I   I    I    I   I 

<i)  Cl—  C—  C—  C—  C—  C—  H,    primary  amyl  chloride; 


H   Cl    H    H    H 

!     I     I     I     I 

(2)  H  —  C  —  C  —  C  —  C  —  C  —  H,    secondary  amyl  chloride. 

I       I       I       I       I 
H    H    H    H    H 

A  different  compound  is  produced  according  to  the 
number  of  carbon  atoms  between  two  introduced  atoms. 
Thus  benzol, 

H 
I 


H— C 


H—  C 


gives  three  dichlorides, 


C 

H 


C— H 


J— H 


— H 


C— H 


Orthochlorbenzol  . 


Metachlorbenzol  . 


C— Cl 


Parachlorbenzol . 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        259 

This  means  that  the  interposition  of  one  or  two  carbon 
atoms  between  those  united  with  the  chlorine  brings  about 
different  properties. 

It  is  not  necessary  to  multiply  examples  further.  It  is 
sufficiently  clear  that  the  nature  of  a  molecule  is  not  merely 
dependent  upon  the  number  and  character  of  the  atoms 
composing  it,  but  is  deeply  modified  by  their  relative 
positions  within  the  molecule. 

Certain  cases  of  isomerism  have  been 

effect  of  Posi-  observed  in  which  the  differences  be- 
tion  in  Space. 

tween  the  bodies  are  physical.    These 

are  known  as  physical  isomers,  and  the  bodies  are  chiefly 
distinguished  from  one  another  by  their  action  upon 
light.  The  usual  explanation  of  the  isomerism,  namely, 
a  different  arrangement  of  the  atoms  as  lying  in  one 
plane,  is  not  possible  in  these  cases.  The  L,e  Bel- van' t 
Hoff  theory  would  explain  these  atoms  or  groups  as 
differently  situated  in  space  of  three  dimensions.  Such 
cases  have  been  observed  only  where  an  asymmetric 
carbon  atom  is  present.  Such  an  asymmetric  atom  is  one 
which  has  each  bond  satisfied  with  a  different  group  or 
atom.  Thus  tartaric  acid  has  the  formula 
H  H 

I  I 

HO— C C— OH. 

C02H         C02H 

Four  modifications  of  this  acid  are  known :  one  polarizing 
light  to  the  right,  one  to  the  left,  one  inactive  form  which 
can  be  resolved  into  the  dextro-  and  laevorotary,  and  one 
which  cannot  be  so  resolved.  Regarding  the  inactive 
form,  which  can  be  resolved  into  the  two  active  forms, 
as  a  mixture  or  combination  of  the  two,  there  are  left 
three  distinct  forms  to  be  accounted  for. 


26O  A   STUDY   OF  THE   ATOMS. 

The  stereochemical  explanation  is  usually  given  as  fol- 
lows :  "  The  'two  immediate  carbon  tetrahedra,  having  a 
common  axis  and  joined  by  one  summit,  have  the  three 
different  groups  arranged  right  or  left.  This  would  re- 
sult in  a  dextro-  and  laevorotary  tartaric  acid.  If,  how- 
ever, the  three  side  groups  are  arranged  in  opposite  di- 
rections, their  influence  will  cease  and  the  product  will 
be  an  inactive  tartaric  acid.  This  cannot  be  resolved."1 

It  may  not  be  absolutely  necessary  to  seek  an  explana- 
tion of  this  isomerism  by  supposing  space  relations  outside 
of  the  plane  surface.  Manifestly  in  such  a  grouping  there 
may  be  three  different  positions  in  the  plane  : 

(I)  (2)  (3) 

H  H  CO2H 

I  I  I 

C—OH  C—  CO2H  C—  H 

I  I  i 

CO2H  OH  OH 

Unless  a  difference  be  granted  in  the  bonds  these  are  the 
only  three  possible  relations.  If  the  kinetic  theory  is 
true,  the  changes  in  the  harmonic  motion  of  the  molecule 
brought  about  by  such  transpositions  might  suffice  to  ac- 
count for  the  slight  changes  in  properties. 

How   are   these   changes  in  the 


cule,  pointed  out  in  the  preceding 
pages,  to  be  explained  ?  The  most  plausible  explanation 
which  has  been  given  is  found  in  the  kinetic  theory. 
Most,  if  not  all,  of  the  properties  of  the  atom  may  be  de- 
pendent upon  its  motion.  This  motion  is  more  or  less  pro- 
foundly modified  by  bringing  it  within  the  sphere  of  in- 
fluence of  another  atom  or  atoms,  and  the  result  is  a  mole- 
cule with  harmonic  molecular  motion  —  a  harmonized  sys- 

1  Richter's  "Organic  Chemistry,"  (Smith)>  p.  475 


MOLECULES  AND  CONSTITUTION  OF  MATTER.   261 

tern  of  motions  evidenced  by  new  properties.  Release  an 
atom  or  a  group  from  this  system,  and  the  old  motion  is 
restored  and  the  former  properties  reappear.  Manifestly 
all  changes  of  relative  position  or  of  grouping  must  mod- 
ify the  motion  of  the  system  and  affect  the  properties.  All 
reproductions  of  the  same  grouping  will  give  the  same 
effect.  The  introduction  of  an  atom  or  group  into  har- 
monically different  parts  of  the  system  will  produce  more 
or  less  distinctly  different  effects.  Without  the  applica- 
tion of  the  kinetic  theory,  these  phenomena  are  exceed- 
ingly difficult  to  explain. 

It  is  impracticable  in  a  work  of  this  com- 

of^olecules  Pass  to  ^^scuss  at  lenStl1  tne  properties 
of  molecules  as  they  have  been  worked 
out  by  the  aid  of  modern  mathematics  and  physics.  It 
will  have  to  suffice  to  enumerate  such  of  these  properties 
as  seem  to  be  more  surely  established.  They  are  of  pro- 
found interest  and  importance  to  science,  but  much  of  the 
work  is  still  too  hypothetical  in  nature.  An  important 
part  of  the  work  of  the  future  will  be  the  thorough  ground- 
ing of  these  theories. 

A  consideration  of  the  general  behavior  of 

gas   molecules,  and   especially   under  the 
Motion.  3  \ 

influence  of  changes  of  temperature,  led 

Herapath,1  Joule,2  Kronig3  and  Clausius*  to  announce  and 
develop  a  mechanical  theory  of  heat  and  a  kinetic  theory 
for  gases.  The  kinetic  theory  is  that  the  molecules  of  a 
gas  are  in  incessant  motion;  this  motion  is  in  a  straight 
line  or  path  and  of  an  unchanging  velocity.  This  kinetic 

1  Annals  of  Phil.,  1821,  pp.  273,  340,  401. 

2  Manchester  Lit.  and  Phil.  Soc.,  1851,  p.  107. 

8  Grundziige  einer  Theorie  der  Gase,  Berlin,  1858. 
*Pogg.  Ann.,  100,353- 


262  A   STUDY   OF   THE   ATOMS. 

theory  is,  in  a  measure,  the  old  vision  of  molecular  motion 
as  seen  by  Greek  philosophers  and  metaphysicians  of  the 
Middle  Ages  reduced  to  a  mathematical  basis. 

These  molecules  meeting  one  another  in  their  paths 
give  rise  to  countless  impacts.  From  the  impacts,  we 
have  the  pressure  or  tension  of  the  gases.  From  this  pres- 
sure the  absolute  velocity  of  the  molecules  has  been  cal- 
culated, and  also  from  the  rates  of  diffusion  of  the  gases. 
The  figures  obtained  show  a  very  great  velocity,  differing 
with  different  gases.  Thus  the  oxygen  molecule  is  said 
to  move  at  a  rate  equal  to  461  meters  per  second  and  the 
hydrogen  molecule  1844  meters  per  second  at  o°  C. 

While  much  work  has  been  done  to 

f.rop_!rjies  .ol  calculate  the  size  of  the  molecule  and 

trie  iVioiecuie.  .  . 

the  position  of  the  component  atoms 

in  space  with  reference  to  the  center  of  gravity,  it  cannot 
yet  be  claimed  that  much  is  definitely  established.  The 
conclusions  of  Meyer1  are,  however,  of  great  interest. 
These  calculations  place  the  diameter  of  a  hydrogen  mole- 
cule at  1.84  millionths  of  a  centimeter.2  The  number  of 
molecules  of  air  in  a  cubic  centimeter  under  a  pressure  of 
one  atmosphere8  is  placed  at  60,000,000,000,000.  It  can 
be  readily  seen  that  it  is  impossible  to  entirely  free  any 
space  of  molecules  of  air  by  means  of  an  air-pump.  A 
large  number  of  molecules  will  still  be  left.  Indeed  the 
number  of  impacts  of  any  one  molecule  upon  other  mole- 
cules is  calculated  as  being  still  46,500  in  a  second  in  a 
volume  of  air  reduced  from  the  pressure  of  one 
atmosphere  to  that  of  o.oi  mm.4  If  all  of  the  mole- 
cules in  a  cubic  centimeter  of  air,  at  ordinary  pres- 

1  "  Kinctische  Theorie  der  Gase,"  pp.  299,  310. 
«/*«.,  IK  3*3- 

3 /£«*.,  p.  335. 
4 /£«/.,  p.  213. 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        263 

sure  be  spread  out  in  a  plane  in  close  contact  with  one 
another,  they  would  cover,  along  with  their  molecular 
spheres,  a  surface  of  1.84  square  meters.1  In  calculating 
the  absolute  weight  of  molecules,  Meyer2  calculated  that 
46,000,000,000,000  molecules  of  air  weigh  one  milligram. 

Efforts  have  been  made  repeatedly  by 
Experimental  M     chemists  and  those  of  more  mod_ 

Investigations. 

ern  times  to  determine  the  limits  of 

divisibility  of  molecules.  These  have  little  value  beyond 
a  rough  confirmation  of  the  preceding  numbers  reached 
by  mathematical  methods.  Thus  Meyer  records  the  ex- 
periment of  A.  W  Hofmann3  that  coloring-matter  can  be 

readily  detected  in  a  dilution  of and  even 

100,000,000 

greater  ;  that  is,  the  smallest  weighable  quantity  can  be 
divided  several  hundred  million  times.  Annaheim4  had 
in  this  way  calculated  that  an  atom  of  hydrogen  must 
weigh  less  than  0.05  millionth  part  of  a  milligram.  There 
is  also  an  experiment  by  Kirchhoff  and  Bunsen,5  which 
shows  that  the  three-millionth  part  of  a  milligram  of  so- 
dium chloride  suffices  to  color  a  flame  distinctly.  Faraday6 
prepared  gold  leaf,  the  thickness  of  which  was  one  hun- 
dred times  less  than  the  length  of  a  light  wave.  Since 
this  leaf  must  at  least  consist  of  a  layer  of  atoms,  the  di- 
ameter of  a  gold  atom  must  be  equal  to  or  less  than  five- 
millionths  of  a  millimeter.  As  noted  on  a  previous  page, 
the  gas  theory  gave  as  the  diameter  of  a  molecule  one- 
fifth  of  a  millionth  of  a  millimeter.  Rontgen7  has  shown 

1  "  Kinetische  Theorie  der  Case,"  p.  301 . 

2  Ibid.,  p.  337. 

3  Ber.  d.  chem.  Ges.,  1870,  p.  660. 

*  Ibid.,  1876,  p.  1151. 

5  Pogg.  Ann.,  1860,  p.  168. 

6  Ibid.,  1857,  p.  318. 

*  Wied.  Ann.,  1890,  41,  321. 


264  A  STUDY  OP  THE  ATOMS. 

that  oil  layers  could  be  prepared  having  a  thickness  of 
only  0.56  millionth  of  a  millimeter.  A  number  of  other 
experiments  are  cited  by  Meyer,  in  which  the  thickness 
of  bubble-films,  the  weights  of  water  films  on  glass,  or  the 
limits  of  capillary  force  were  determined  in  the  effort  at 
settling  the  limits  of  molecular  diameters.  These  approx- 
imations tend  to  confine  the  estimate  given  above. 

Since  the  diffraction  of  light  in  the  microscope  prevents 
a  clear  definition  of  anything  smaller  than  the  one  four- 
thousandth  part  of  a  millimeter,  no  direct  use  can  be 
made  of  this  instrument  in  the  investigation  under  con- 
sideration. Still  there  are  optical  methods  which  have 
been  used,1  giving  results  that  coincide  well  with  those 
deduced  from  the  theory  of  gases.  Electrical  methods 
have  been  used  by  Thomson,  by  L,orenz,s  and  by  Ober- 
beck.3 

Meyer  draws  the  conclusion*  that  while  these  methods 
for  determining  the  limit  of  divisibility  of  matter  do  not 
all  yield  similar  results  as  to  the  size  of  the  particles,  yet 
they  agree  without  exception  in  this  that  the  thickness  of 
a  molecule  of  the  material  examined  can  not  be  less  than 
the  millionth  part  of  a  millimeter.  He  regards  this  as  a 
fairly  well-determined  limit  of  size  for  the  smallest  parti- 
cles. 

It  might  be  claimed  that  none  of  these  methods  give 
any  direct  proof  of  the  existence  of  particles  at  all  but 
simply  concern  the  thickness  of  material.  One  experi- 
ment of  Sir  William  Thomson  would  seem  to  meet  this 
objection.  This  was  referred  to  above.1  The  simple 
laws  of  dispersion  in  transparent  substances  could  not  be 

1  W.  Thomson  in  Bxner's  Report,  ai,  222  (1885). 

2  Pogg.  Ann.,  140,  644  (1870). 
8  Wied.  Ann.,  31,  337  (1887). 
*  Meyer  :  Loc.  cit. ,  342 . 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        265 

true  if  only  a  few  particles  were  found  in  the  path  of  a 
light  wave.  If  there  are  many  of  these  present,  then  the 
distance  between  two  neighboring  molecules  must  be 
much  smaller  than  the  length  of  a  light  wave.  If  the 
number  of  these  is  1,000,  then  we  get  for  the  value  of  the 
distance  0.000,000,5  mm.,  a  number  which  agrees  with 
that  obtained  from  a  consideration  of  the  kinetic  theory. 

There  still  remains  the  old  puzzle  as 

'  to  the  divisibility  of  matter>  or  rather, 
as  Meyer  puts  it,  '  'as  to  the  marvelous 
property  of  indivisibility."  Something  has  been  learned 
as  to  the  size,  weight,  form  and  motion  of  the  molecules. 
These  we  suppose  to  be  made  up  of  atoms,  and  no  diffi- 
culty is  experienced  in  separating  them  into  their  com- 
ponent atoms.  There  are  indications  that  the  atoms 
themselves  are  related,  have  some  common  constituents, 
and  so  are  compound,  but  the  problem  of  their  division 
remains  unsolved.  The  calculations  just  given  as  to  their 
size  would  make  it  also  extremely  improbable  that  such 
relatively  large  bodies  are  really  indivisible.  Repeated 
efforts  have  been  made  to  split  up  these  atoms,  but  the 
lines  of  investigation  have  promised  little  and  yielded 
nothing.  There  are,  however,  several  hypotheses  as  to 
the  nature  of  atoms  which  are  of  interest,  though  of  course 
little  weight  can  be  attached  to  them  in  their  unsupported 
condition. 

,  This  hypothesis  was  attributed  by  Ran- 

Hypothesls.  kinel  tO  Sir  HumPhry  Dayy-  .  Rankine 
was,  however,  the  first  to  develop  it  by 
mathematical  methods.  It  is  an  hypothesis  of  molecular 
vortices  which  assumed  ' '  that  each  atom  of  matter  con- 
sists of  a  nucleus  or  central  point  enveloped  by  an  elastic 

i  Phil.  Mag.,  10,  354,  411  (1855). 


266  A   STUDY   OF   THE   ATOMS. 

atmosphere,  which  is  retained  in  its  position  by  attractive 
forces,  and  that  the  elasticity  due  to  heat  arises  from  the 
centrifugal  force  of  those  atmospheres  revolving  or 
oscillating  about  their  nuclei  or  central  points. ' '  Whether 
these  elastic  atmospheres  are  continuous  or  consist  of  dis- 
crete particles,  Rankine  does  not  attempt  to  decide. 

The  vortex  theory  of  Thomson1  is  based 
Vortex  upon  a  mathematical  investigation  of  Helm- 

holtz  in  which  the  vortex  motion  of  a  fluid 
in  motion  without  friction  was  examined.  If  we  take  the 
rings  of  smoke,  such  as  are  sometimes  observed,  we  shall 
find  in  them  an  illustration  of  the  vortices.  Helmholtz, 
assuming  the  fluid  to  be  incompressible,  homogeneous 
and  without  friction,  proved  by  mathematical  methods : 

(1)  That  if  such  a  vortex  is  once  formed,  it  will  con- 
tinue to  exist  forever.     It  cannot  be  destroyed  in  such  a 
medium,  nor  produced.     It  required  an  act  of  creation  at 
the  time  of  formation  of  the  liquid. 

(2)  A  vortex   always  consists  of  the  same  portion  of 
the  fluid.     It  is  not  mere  motion  in  the  fluid,  but  actual 
transference  or  traveling  of  the  same  portion  of  the  fluid. 

(3)  No  two  vortices  can  occupy  the  same  space  nor  in- 
tersect one  another.    A  vortex  must  behave  as  a  perfectly 
elastic  body. 

Other  important  deductions  were  made  by  Helmholtz, 
but  these  are  the  ones  most  directly  applied  by  Thomson 
in  his  theory. 

The  theory  of  Thomson  has  to  this  ex- 
£jj°™son's  tent  connection  with  the  Cartesian  theory 

in  that  all  space  is  supposed  to  be  filled 
with  continuous,  homogeneous,  frictionless  matter  which 
has  the  nature  of  a  fluid  and  is  like  the  ether  of  ancient  and 

i  Phil.  Mag.,  34,  15  (1867). 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        267 

modern  physicists.  There  is  but  this  one  kind  of  matter. 
Out  of  this  continuous  mass,  small  ring- like  portions  sep- 
arate. These  cannot  separate  because  of  any  motion  in 
the  ether  itself.  They  cannot  be  divided  into  parts,  nor 
can  they  be  destroyed  by  any  force  originating  in  matter 
made  up  of  them.  These  vortices  are  the  atoms  of  all 
ponderable  substances,  and  between  them  lies  the  original 
ether.  ' '  The  unchanging  mass  of  these  vortex  atoms  is 
determined  solely  by  the  condition  of  the  motion  in  which 
the  world  found  itself  at  its  creation.  The  manifold  char- 
acter of  these  conditions  had  called  forth  manifold  kinds 
of  vortices,  which,  in  spite  of  this,  were  built  up  of  the 
same  substance  and  according  to  the  same  laws,  and 
which  must  bear  witness  to  these  laws  for  all  time  by  the 
regularity  of  their  characteristics.  Thus  would  this 
theory  make  it  possible  to  explain  the  obedience  to  law 
shown  by  the  properties  of  the  atoms,  and  especially  to 
the  law  of  the  periodicity  of  these  properties."1 

In  considering  this  theory  Maxwell  says:2  "when  the 
vortex  atom  is  once  set  in  motion,  all  its  properties  are 
absolutely  fixed  and  determined  by  the  laws  of  motion  of 
the  primitive  fluid,  which  are  fully  expressed  in  the  fun- 
damental equation.  The  disciple  of  Lucretius  may  cut 
and  carve  his  solid  atoms  in  the  hope  of  getting  them  to 
combine  into  worlds  ;  the  followers  of  Boscovich  may  im- 
agine new  laws  of  force  to  meet  the  requirements  of  each 
new  phenomenon,  but  he  who  dares  to  plant  his  feet  in  the 
path  opened  up  by  Helmholtz  and  Thomson  has  no  such 
resources.  His  primitive  fluid  has  no  other  properties 
than  inertia,  invariable  density,  and  perfect  mobility,  and 
the  method  by  which  the  motion  of  this  fluid  is  to  be 
traced  is  pure  mathematical  analysis.  The  difficulties  of 

1  Meyer,  "  Kinetische  Theorie  der  Case,"  p.  351. 

2  Encyc.  Brit.  Article  Atom. 


268  A   STUDY   OF   THE   ATOMS. 

this  method  are  enormous  but  the  glory  of  surmounting 
them  would  be  unique.  '  ' 

These  vortex-atoms  must  be  perfectly 
elastic'  even  thou£h  the  ether  itself  be 


the 

devoid  of  elasticity.     In  the  case  of  im- 

pacts, these  atoms  would  behave  in  a  manner  similar  to 
elastic  bodies,  and  it  is  easy  to  see  how  light,  swinging 
movements  of  the  atoms  would  be  transmitted  to  the  ether 
and  from  the  ether  to  the  atoms.  Thus  an  influence  can 
be  exerted  by  atom  upon  atom  at  a  distance.  Thomson 
and  Tait,1  Kirchhoff8  and  others  have  shown  mathemati- 
cally how  rings  and  other  bodies  which  are  in  a  fluid  in 
motion  exercise  an  influence  apparently  comparable  to  an 
electrodynamic  upon  one  another. 

The  vortex  need  not  have  the  form  of  rings.  The 
rings  may  be  knotted  (without  intersection),  or  other 
forms  can  be  supposed.  Pulsating  masses  have  been 
considered  which,  having  a  spherical  or  similar  form, 
show  an  internal  motion  in  which  at  any  one  point  there 
are  regular  vibrations  in  a  radial  direction. 

It  is  by  rigid  mathematical  analysis 
Consequences  of  that  the  vortex  theory  and  its  con- 
the  Theory.  ,  j  Ti 

sequences  are  to  be  worked  out.    It 

admits  of  few  assumptions.  It  is  most  closely  connected 
with  the  theory  of  electricity  and  light.  It  means  not 
merely  a  kinetic  theory  of  gases  but  of  solids  and  liquids, 
of  heat,  light  and  electricity.  The  harmony  of  the  uni- 
verse is  motion,  and  so  at  the  close  of  more  than  twenty 
centuries  we  come  back  to  a  theory  of  a  universe  filled 
with  a  continuous  matter,  and,  at  the  same  time,  an 
atomic  theory.  But  the  theory  is  no  longer  a  baseless 

1  Treatise  on  Nat.  Phil.,  i,  264  (1867). 

2  Crelle  :  Borchardfsjour.,  71,  137,  263  (1870). 


MOLECULES  AND  CONSTITUTION  OF  MATTER.       269 

dream.  It  would  seem  to  be  the  culmination  of  centuries 
of  work,  not  fancy,  and  to  embody  the  explanation  of  all 
facts  known  —  chemical,  physical  and  mathematical.  There 
is  still  much  to  be  done  and  many  untrodden  patfis.  The 
theory  must  yet  stand  many  exacting  tests,  but  so  far  at 
least  nothing  has  been  thought  out  which  so  satisfies  the 
conditions  known  to  us. 

It    will    readily    be   seen   that 


. 

atoms  agrees  well  with  the 
kinetic  equilibrium  theory  of  valence  and  offers  a  satis* 
factory  explanation  of  the  difference  between  the  atoms. 
In  the  case  of  a  univalent  atom,  we  have  a  vortex  whose 
motion  enables  it  to  enter  into  harmonic  motion  with  one 
other  vortex,  giving  a  stable  molecule  ;  for  a  bivalent  atom, 
the  motion  is  such  that  there  can  be  unison  with  two  of  the 
former  vortices  or  with  one  having  a  similar  motion.  This 
motion  may  be  dependent  upon  the  peculiar  form  of  the 
vortex.  Thus,  elementary  atoms  of  Group  I  might  have 
one  distinctive  form  and  motion,  of  Group  II  another,  and 
so  on  through  Group  IV,  or  possibly  through  Group  VII. 
A  change  of  valence,  which  we  have  seen  was  so  easily 
brought  about  by  the  action  of  another  force,  as  heat  or 
light,  would  mean  a  change  of  form  and  motion  in  the 
vortex.  Thus  a  vortex  with  three  knots  might  become  a 
simple  ring  or  a  vortex  having  a  different  number  of 
knots.  It  is  evident,  however,  that  there  is  some  ten- 
dency to  return  to  the  original  form  and  motion  when  the 
original  conditions  are  restored. 

The  motion  of  the  vortex  atom 

Thn  Y«-te-*  The°fy        makes  it  a  center  of  force.  There 
and  Affinity. 

is  no  force  without  motion.     The 

motionless  ether  is  without  force.     Weight,  which  is  but 


270  A   STUDY   OF  THE   ATOMS. 

one  form  of  attraction,  acting  at  a  distance  and  dependent 
upon  mass,  is  one  of  the  results  of  this  force.  The  ether 
is  without  weight.  The  properties  of  the  atoms  show  a 
certain  periodicity  according  to  mass  and  weight,  that  is, 
are  determined  by  the  motion  of  the  vortex  ;  chemical 
affinity  is  another  kind  of  attraction  which  must  also  de- 
pend upon  the  motion,  and  in  some  way  may  be  related  to 
the  motion  of  electricity.  What  is  meant  by  the  union  of 
atoms  other  than  the  joining  of  two  or  more  vortices  in 
harmonic  motion  is  unknown  to  us,  but  the  new  motion 
of  the  harmonic  system  means,  of  course,  new  properties 
depending  upon  this  motion.  The  dissociation  of  this 
molecule  restores  the  old  condition  of  motion  and  the 
properties  dependent  upon  it.  The  laws  of  distribution 
of  acids  and  bases  in  double  decompositions  and  of  mass 
action  in  general  should  afford  valuable  data  for  reducing 
to  a  rigid  mathematical  basis  these  questions  of  motion 
and  form. 

J.  J.  Thomson  has  announced  a  hypoth- 
H  C  thes's  es*S  w*"cn'  while  referring  more  di- 

rectly to  force,  has  its  bearing  ultimately 
upon  the  constitution  of  matter.  The  basis  of  the 
hypothesis  is  decidedly  debatable,  presenting  points 
which  may  not  be  generally  admitted.  The  hypothesis 
itself  is  used  to  explain  certain  phenomena  connected  with 
electricity  and  those  emanations  of  force  or  matter  known 
as  Rontgen  rays,  Becquerel  rays,  etc. 

It  is  deduced1  from  Faraday's  laws  of  electrolysis  that 
the  current  through  an  electrolyte  is  carried  by  the  atoms 
of  the  electrolyte,  and  that  all  of  these  atoms  carry  the 
same  charge,  so  that  the  weight  of  the  atoms  required  to 
carry  a  given  quantity  of  electricity  is  proportional  to  the 

1  Pop.  Set.  Monthly,  1901,  p.  323. 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        271 

quantity  carried.  To  carry  the  unit  charge  of  electricity 
requires  a  collection  of  atoms  of  hydrogen  which  together 
weigh  about  o.  i  milligram.  If  the  charge  of  electricity 
on  an  atom  of  hydrogen  can  be  measured  then  one-tenth 
of  this  charge  (numerically)  will  be  the  weight  of  the 
atom  of  hydrogen  in  milligrams.  Thomson  shows  how 
this  charge  may  be  measured.  To  carry  a  given  charge 
of  electricity  by  hydrogen  atoms  requires  a  mass  a  thousand 
times  greater  than  to  carry  it  by  the  negatively  electrified 
particles,  which  constitute  the  cathode  rays,  and  it  is  very 
significant  that  while  the  mass  of  atoms  required  to  carry 
a  given  charge  through  a  liquid  electrolyte  depends  upon 
the  kind  of  atom,  being,  for  example,  eight  times  greater 
for  oxygen  than  for  hydrogen  atoms,  the  mass  of  cathode 
ray  particles  required  to  carry  a  given  charge  is  quite  in- 
dependent of  the  gas  through  which  the  rays  travel  and 
of  the  nature  of  the  electrode  from  which  they  start.  By 
a  very  ingenious  method  it  seems  possible  to  determine 
the  electric  charge  carried  by  one  of  these  particles.  The 
conclusion  is  reached  that  the  charge  on  one  of  these 
particles  is  the  same  as  that  on  an  atom  of  hydrogen  in 
electrolysis.  From  this  it  follows  that  the  mass  of  each 
of  these  particles  is  only  about  one  one-thousandth  part  of 
a  hydrogen  atom.  These  negatively  electrified  particles 
Thomson  calls  corpuscles.  They  form  an  invariable  con- 
stituent of  the  atoms  or  molecules  of  all  gases,  and  pre- 
sumably of  all  liquids  and  solids.  These  corpuscles  seem 
to  be  given  off  by  incandescent  metals  and  by  certain 
radioactive  bodies.  The  carriers  of  negative  electricity 
are  these  corpuscles  of  invariable  mass.  The  carriers  of 
positive  electricity  are  connected  with  a  mass,  which  is  of 
the  same  order  as  that  of  an  ordinary  molecule  and  which 
varies  with  the  nature  of  the  gas  in  which  the  electrifica- 


272  A   STUDY   OF  THE   ATOMS. 

tion  is  found.  Thomson  conceives  that  negative  elec- 
tricity consists  of  these  corpuscles,  and  that  positive  elec- 
trification consists  in  the  absence  of  these  corpuscles  from 
ordinary  atoms.  Negative  electricity  (/.  <?.,  the  electric 
fluid)  has  mass  ;  a  body  negatively  electrified  has  a 
greater  mass  than  the  same  body  in  the  neutral  state ; 
positive  electrification,  since  it  involves  the  absence  of 
corpuscles,  is  accompanied  by  a  diminution  in  mass.  The 
idea  that  mass  in  general  is  electrical  in  its  origin  is  a 
fascinating  one  to  Thomson,  although  he  acknowledges 
that  it  has  not  at  present  been  reconciled  with  the  results 
of  experience. 

Of  course  these  corpuscles,  if  their  existence  can  be 
surely  maintained,  have  a  most  important  bearing  upon 
the  constitution  of  matter.  If  his  suggestions  are  true 
that  electricity  has  weight  and  mass — that  mass  is  elec- 
trical— then  the  ultimate  conclusion  is,  that  force  occupies 
space  and  there  is  no  matter.  Force  alone  makes  up  the 
Universe. 

It  can  only  be  said  that  satisfactory  evidence  is  lacking 
and  the  conclusion  unjustified  at  present. 

The  following  summary  by  Crookes1  is 

c  °  given  in  a  condensed  form  here  to  show 

Nummary. 

the  views  held  by  one  who  has  taken  a 

somewhat  advanced  stand  in  speculation  as  to  chemical 
theory. 

"  For  nearly  a  century,  men  who  devote  themselves  to 
science  have  been  dreaming  of  atoms,  molecules,  ultra- 
mundane particles  and  speculating  as  to  the  origin  of 
matter.  To  show  how  far  we  have  been  propelled  on  the 
road,  we  have  but  to  recall  matter  in  a  fourth  state,  the 
genesis  of  the  elements,  the  existence  of  bodies  smaller 

1  Science,  17,  993. 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        273 

than  atoms,  the  atomic  nature  of  electricity,  and  the  per- 
ception of  electrons. 

"  In  1879  I  advanced  the  theory  that  in  the  phenomena 
of  the  vacuum  tube  at  high  exhaustions  the  particles  con- 
stituting the  cathode  stream  are  not  solid,  not  liquid,  nor 
gaseous,  do  not  consist  of  atoms  propelled  through  the 
tube  and  causing  luminous  mechanic  or  electric  pheno- 
mena where  they  strike,  but  that  they  consist  of  some- 
thing much  smaller  than  the  atom — fragments  of  matter, 
ultra-atomic  corpuscles,  minute  things  very  much  smaller, 
very  much  lighter  than  atoms — things  which  seem  to  be 
the  foundation  stones  of  which  atoms  are  composed. 

"  In  1888  in  connection  with  a  theory  of  the  genesis  of 
the  elements  I  spoke  of  an  infinite  number  of  immeasura- 
bly small  ultimate  particles  gradually  accreting  out 
of  the  formless  mist  and  moving  with  inconceivable 
velocity  in  all  directions.  I  strove  to  show  that 
the  elementary  atoms  themselves  might  not  be  the 
same  now  as  when  first  generated,  that  the  primary 
motions  which  constitute  the  existence  of  the  atom 
might  slowly  be  changing  and  even  the  secondary 
motions  which  produce  all  the  effects  we  can  observe 
— heat,  chemic,  electric,  and  so  forth — might  in  a  slight 
degree  be  affected  and  the  probability  was  shown  that  the 
atoms  of  the  chemical  elements  were  not  eternal  in  ex- 
istence, but  shared  with  the  rest  of  creation  the  attributes 
of  decay  and  death. 

"  Another  phase  of  the  dream  now  demands  attention. 
W.  K.  Clifford  said  in  1875  :  '  There  is  great  reason  to 
believe  that  every  material  atom  carries  upon  it  a  small 
electric  current,  if  it  does  not  wholly  consist  of  this  cur- 
rent.' 

'  The  idea  of  unit  or  atoms  of  electricity  which   has 


274  A   STUDY   OF  THE   ATOMS. 

been  contributed  to  by  Faraday  and  others,  took  con- 
crete form  when  Stoney  showed  that  Faraday's  law  of 
electrolysis  involved  the  existence  of  a  definite  charge  of 
electricity  associated  with  the  ions  of  matter.  This  defi- 
nite charge  he  called  an  electron.  It  was  not  till  some 
time  after  the  name  had  been  given  that  electrons  were 
found  to  be  capable  of  existing  separately. 

"  During  my  inaugural  address  in  1891  as  president  of 
the  Institution  of  Electrical  Engineers  an  experiment  was 
shown  which  went  far  to  prove  the  dissociation  of  silver 
into  electrons  and  positive  atoms.  A  silver  pole  was  used 
and  near  it  in  front  was  a  sheet  of  mica  with  a  hole  in  its 
center.  The  vacuum  was  very  high  and  when  the  poles 
were  connected  with  the  coil,  the  silver  being  negative, 
electrons  shot  from  it  in  all  directions  and,  passing  through 
the  hole  in  the  mica  screen,  formed  a  bright  phosphores- 
cent patch  on  the  opposite  side  of  the  bulb.  Silver  was 
seen  to  be  deposited  on  the  mica  screen  only  in  the  im- 
mediate neighborhood  of  the  pole,  the  far  end  of  the 
bulb,  which  had  been  glowing  for  hours  from  the  impact 
of  electrons,  being  free  from  silver  deposit.  Here,  then, 
are  two  simultaneous  actions.  Electrons,  or  radiant 
matter,  shot  from  the  negative  pole,  caused  the  glass 
against  which  they  struck  to  glow  with  the  phosphores- 
cent light.  Simultaneously,  the  heavy  positive  ions  of 
silver  freed  from  negative  electrons  and  under  the  in- 
fluence of  the  electrical  stress  likewise  flew  off  and  were 
deposited  in  the  metallic  state  near  the  pole.  The  ions 
of  metal  thus  deposited  in  all  cases  showed  positive  elec- 
trification. 

"All  of  the  isolated  facts  mentioned — ultragaseous 
matter,  division  of  atoms,  electrons,  etc.,  are  focused  and 
welded  into  one  harmonious  theory  by  the  discovery  of 


MOLECULES  AND  CONSTITUTION  OF  MATTER.    275 

radium.  L,et  me  briefly  recount  some  of  the  properties 
of  radium  and  show  how  it  reduces  speculations  and 
dreams,  apparently  impossible  of  proof,  to  a  concrete 
form. 

' '  The  most  striking  property  of  radium  is  its  power  to 
pour  out  torrents  of  emanations  which  are  of  three  kinds. 
One  set  is  the  same  as  the  cathode  stream,  now  identified 
with  free  electrons.  These  electrons  are  neither  ether 
waves  nor  a  form  of  energy,  but  substances  possessing 
inertia  (probably  electric) .  Liberated  electrons  are  ex- 
ceedingly penetrating.  They  will  discharge  an  electro- 
scope when  the  radium  is  10  feet  or  more  away,  and  will 
affect  a  photographic  plate  through  5  or  6  mm.  of  lead. 
They  are  not  readily  filtered  out  by  cotton  wool.  They 
do  not  behave  as  a  gas,  but  more  like  a  fog  or  mist. 
They  are  deviable  in  a  magnetic  field.  They  are  shot 
from  radium  with  a  velocity  of  about  one-tenth  that  of 
light  but  are  gradually  obstructed  by  collisions  with  air 
atoms  so  that  some  become  much  slowed  and  then  diffuse 
in  the  air  and  give  it  temporary  conducting  powers. 

"Another  set  of  emanations  from  radium  are  not  affected 
by  an  ordinary  powerful  magnetic  field  and  are  incapable 
of  passing  through  thin  material  obstructions.  These 
have  about  one  thousand  times  the  energy  of  those  radiated 
by  the  deflectable  particles.  They  render  air  a  conductor 
and  act  strongly  on  a  photographic  plate.  Their  mass  is 
enormous  compared  with  that  of  the  electrons  and  their 
velocity  is  probably  as  great  when  they  leave  the  radium, 
but  in  consequence  of  their  greater  mass  they  are  less  de- 
flected by  the  magnet,  are  easily  obstructed  by  obstacles 
and  are  sooner  brought  to  rest  by  collisions  with  air 
atoms.  These  are  affirmed  to  be  the  positive  ions.  Ruther- 
ford has  shown  that  these  emanations  are  slightly  affected 


276  A  STUDY   OF   THE   ATOMS. 

in  a  very  powerful  magnetic  field  but  in  an  opposite 
direction  to  the  negative  electrons.  He  has  measured 
their  speed  and  mass  and  shown  them  to  be  ions  of 
matter  moving  with  the  speed  of  the  order  of  that  of 
light. 

* '  There  is  also  a  third  kind  of  emanation  produced  by 
radium.  These  accompany  the  others.  They  are  not  at 
all  affected  by  magnetism  and  are  Rontgen  rays — ether 
vibrations — produced  as  secondary  phenomena  by  the 
sudden  arrest  of  velocity  of  the  electrons  by  solid 
matter. 

* '  The  actions  of  these  emanations  on  phosphorescent 
screens  are  different.  The  electrons  are  much  less  pene- 
trating than  Rontgen  rays.  The  power  with  which 
radium  emanations  are  endowed  of  discharging  electrified 
bodies  is  due  to  the  ionization  of  the  gas  through  which 
they  pass.  This  can  be  affected  in  many  other  ways  as 
by  splashing  water,  red  hot  bodies,  flame,  etc. 

''According  to  Sir  Oliver  Lodge's  electronic  theory,  an 
atom  of  matter  has  a  few  extra  negative  electrons  in  addi- 
tion to  the  neutral  atom.  When  these  are  removed,  it 
becomes  positively  charged.  The  negative  charge  con- 
sists of  unbalanced  electrons,  one,  two,  three,  etc.,  accord- 
ing to  the  balance. 

1  *  It  is  recognized  that  the  electrons  have  the  one  prop- 
erty which  has  been  regarded  as  inseparable  from  matter, 
namely  inertia.  In  1881  J.  J.  Thomson  developed  the 
idea  of  electric  inertia  (self-induction)  due  to  a  moving 
charge.  The  electron,  therefore,  appears  only  as  appar- 
ent mass  by  reason  of  its  electrodynamic  properties,  and 
if  we  consider  all  forms  of  matter  to  be  merely  congeries 
of  electrons,  the  inertia  of  matter  would  be  explained 
without  any  material  basis." 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        277 

In  the  Romanes  Lecture  for  June 


certain  views  as  to  the  nature  of 
matter.  He  stated  as  his  first  thesis  "  generally  accepted 
by  physicists"  that  an  electric  charge  possessed  the  funda- 
mental property  of  matter,  called  mass  or  inertia,  and  that 
if  a  charge  were  sufficiently  concentrated  it  might  repre- 
sent any  amount  of  matter  desired.  There  were  reasons 
for  supposing  that  electricity  existed  in  such  concentrated 
small  portions,  which  were  called  electrons,  and  could 
either  be  associated  with  atoms  of  water,  to  form  the  well- 
known  chemical  ions  or  could  fly  separate  as  was  observed 
in  the  cathode  rays  of  vacuum  tubes  and  in  the  loss  of 
negative  electricity  when  ultraviolet  light  falls  upon  a 
clean  negatively  charged  surface.  The  hypothesis  sug- 
gested on  the  strength  of  these  facts  is,  that  the  atoms  of 
matter  are  actually  composed  of  these  unit  electric  charges 
or  electrons,  an  equal  number  of  positive  and  negative 
charges  going  to  form  a  neutral  atom,  a  charged  atom 
having  one  electron  in  excess  or  defect.  On  this  view  a 
stable  aggregate  of  about  700  electrons  in  violent  orbital 
motion  among  themselves  would  constitute  a  hydrogen 
atom,  sixteen  times  that  number  would  constitute  an 
oxygen  atom,  and  about  150,000  would  constitute  an  atom 
of  radium. 

This  hypothesis  represents  a  unification  of  matter  and 
a  reduction  of  all  material  substances  to  purely  electrical 
phenomena.  Assuming  this  electrical  theory  of  matter, 
that  the  atoms  are  aggregates  of  electric  charges  in  a 
violent  motion,  two  consequences  follow.  One  of  these 
consequences  depends  on  the  known  fact  that  radiation 
or  light  or  an  ether  wave  of  some  kind,  is  emitted  from 

1  Science,  18,  122. 


278  A   STUDY    OF  THE   ATOMS. 

any  electron  subject  to  acceleration  ;  consequently  the  re- 
volving constituents  of  an  atom  must  be  slowly  radiating 
their  energy  away,  must  thus  encounter  a  virtual  resist- 
ance and  must  in  that  way  have  their  velocity  increased. 

The  second  consequence  is  that  when  the  speed  of  an 
electrified  body  reaches  that  of  light  its  mass  becomes 
suddenly  infinite  ;  and  in  that  case  it  appears  not  im- 
probable that  a  critical  condition  would  have  been  reached, 
at  which  the  atom  would  no  longer  be  stable  but  would 
break  up  into  other  substances.  And  recently  a  break-up 
of  the  most  massive  atoms  has  been  observed  by  Ruther- 
ford, and  has  been  shown  to  account  for  the  phenomenon 
of  radioactivity,  some  few  of  the  atoms  of  a  radioactive 
substance  appearing  to  reach  a  critical  stage  at  which 
they  fling  away  a  small  portion  of  themselves  with  great 
violence,  the  residue  having  the  same  property  of  insta- 
bility for  some  time,  until  ultimately  it  settles  down  into 
presumably  a  different  substance  from  that  which  it  was 
at  the  beginning. 

Lodge's  further  hypotheses  and  speculations  may  well 
be  omitted  here. 

It  is,  of  course,  impossible,  in  the 
H  th01"  S  present  state  of  knowledge  concerning 
the  radioactive  bodies  and  their  strange 
emanations,  to  attach  any  serious  value  to  the  various 
guesses  at  a  solution  of  the  problems  involved.  The 
phenomena  are  so  new  and  so  remarkable  that  former  ex- 
perience can  not  serve  us,  and  some  of  the  older  hypothe- 
ses or  ideas  as  to  matter  and  force  will  apparently  require 
an  overhauling  and  reformulation.  In  closing  the  subject 
Rutherford's  suggestions1  may  be  mentioned  as  at  least 
interesting. 

i  Rutherford  :  "Disintegration  of  the  Radioactive  Elements,"  Harpers  Mag., 
1904,  p.  280. 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        279 

' '  The  radiating  power  is  thus  an  inherent  property  of 
the  radioactive  elements  and  must  reside  in  the  atoms 
themselves.  Since  the  radiation  consists  of  the  projection 
of  matter,  this  matter  must  be  a  part  of  the  atom  and 
the  latter  must  suffer  disintegration.  Now  it  is  impossi- 
ble to  imagine  any  mechanism  possessed  by  the  heavy 
atoms  of  the  radioactive  elements  whereby  they  suddenly 
project  from  rest  a  portion  of  themselves  with  enormous 
velocity.  It  seems  far  more  likely  that  the  atoms  them- 
selves are  very  complex  systems,  consisting  of  smaller 
charged  parts  in  rapid  rotation  and  held  in  equilibrium  by 
their  mutual  forces.  For  some  reason  the  atom  becomes 
unstable,  and  one  of  these  parts  suddenly  escapes  from  the 
system  with  the  velocity  it  possessed  in  its  orbit.  1.  .  . 
The  chain  of  substances  that  are  being  spontaneously 
produced  from  the  parent  element  cannot  be  due  to  the 
breaking  up  of  molecular  systems  but  must  arise  from  an 
actual  disintegration  of  the  atoms  of  the  radioactive  ele- 
ments into  simpler  forms. ' ' 

It  is  easy  to  see  that  much  of  the 
-Peculation  of  the  last  few  pages  is 
based  on  very  questionable  evidence 
or  altogether  unsupported  by  fact.  It  is  well  to  quote  in 
conclusion  the  conservative  words  of  Clarke  in  the  Wilde 
lecture  delivered  on  the  centennial  anniversary  of  the  an- 
nouncement of  the  atomic  theory.1  "If  we  take  the 
atomic  theory  out  of  chemistry  we  shall  have  left 
but  a  dust  heap  of  unrelated  facts.  The  convergence  of 
the  testimony  is  remarkable  and  when  we  add  to  the 
chemical  evidence  that  which  is  offered  by  physics  the 
theory  becomes  overwhelmingly  strong.  And  yet,  from 
time  to  time,  we  are  told  that  the  theory  has  outlived  its 

1  (Manchester  literary  and  Philosophical  Society,  Vol.  47,  N.  n.) 


2 80  A  STUDY   OF   THE   ATOMS. 

usefulness  and  that  it  is  now  a  hindrance  rather  than  a 
help  to  science.  When  we  say  that  matter  as  we  know 
it,  behaves  as  if  it  were  made  up  of  very  small,  discrete 
particles,  we  do  not  lose  ourselves  in  metaphysics  and  we 
have  a  definite  conception  which  can  be  applied  to  the 
correlation  of  evidence  and  the  solution  of  problems.  Ob- 
jections count  for  nothing  against  it  until  something  better 
is  offered  in  its  stead,  a  condition  which  the  critics  of  the 
atomic  theory  have  so  far  failed  to  fulfil. 

' '  Up  to  a  certain  point  we  can  easily  dispense  with  the 
atomic  theory,  for  we  can  start  with  the  fact  that  every 
element  has  a  definite  combining  number  and  then  with- 
out any  assumption  as  to  the  ultimate  meaning  of  these 
constants,  we  can  show  that  other  constants  are  intimately 
connected  with  them.  So  far  we  can  ignore  the  origin  of 
the  so-called  atomic  weight  ;  but  the  moment  we  en- 
counter the  facts  of  isomerism  or  chemical  structure,  and 
of  the  partial  substitution  of  one  element  by  another,  our 
troubles  begin.  The  atomic  theory  connects  all  of  these 
data  together  and  gives  the  mind  a  simple  reason  for  the 
relations  which  are  observed.  We  cannot  be  satisfied 
with  mere  equations ;  our  thoughts  will  seek  for  that 
which  lies  behind  them." 

A  suggestive  "attempt  at  a  chemical 
conception  of  universal  ether"  has 
been  published  lately  by  Mendeleeff. 
This  is  speculation,  of  course,  but  coming  from  the  dis- 
tinguished author  of  the  Periodic  System  is  well  worthy 
of  consideration.  The  chief  reason  for  mentioning  it  here, 
however,  is  because  it  contains  an  interesting  modifica- 
tion of  his  original  Periodic  Table  of  the  Elements. 
Mendeleeff 's  propositions  with  regard  to  this  ether,  which 
permeates  all  bodies  and  fills  all  space,  are  as  follows : 


MOLECULES  AND  CONSTITUTION  OF  MATTER.        28 1 

1 .  It  must  have  weight,  or  mass,  if  it  is  matter. 

2.  Reasoning  from  its  power  of  permeating  all  bodies 
and  from  its  possible  analogy  to   argon   and  its  com. 
panions,  he  would  think  of  ether  as  an  inert  gas  incapable 
of  combination. 

3.  He  does  not  conceive  of  the  other  elements  as 
formed  from  this  and  sees  no  simplification  in  a  common 
origin  of  the  elements.     Unity  of  a  higher  order  is  given 
by  the  conception  of  ether  as  the  final  link  in  the  chain 
of  elements. 

4.  He  forms  a  new  group,   therefore   with  ether  =  X 
and  coronium  =  Y  and  then  helium,  argon,  etc.     This 
group  is  O.     From  considerations  of  molecular  velocity 
he  attempts  to  calculate  the  limits  for  the  atomic  weight 
of  ether. 

5.  He  believes   that  radioactivity   indicates  a  material 
emanation  and  that  the  arrival  and  departure  of  ether 
atoms  are  accompanied  by  the  disturbances  which  con- 
stitute waves  of  light.     The  chief  cause  of  the  sun's 
luminosity  is  its  great  mass,   and  the  accumulation  of 
ether  due  to  its  attraction.     And  thus  ether  is  attracted 
by  the  great  mass  of  the  atom  of  uranium,  thorium, 
radium,  etc.,  explaining  the  radioactivity. 

The  table  follows: 


282 


A  STUDY  OF 
PERIODIC  TABLE 


THE  ATOMS. 
OF  ELEMENTS. 


Series. 

Group  O. 

Group  I. 

Group  H.fc 

Group  III. 

Group  IV. 

Group  V. 

> 
a 

0 

Group  VII. 

> 

CU 

o 

O 

X 

H 

I 

y 

1.008 

He 

1*1 

Be 

B 

C 

N 

o 

F 

2 

4 

7.03 

9-1 

II 

12 

14.04 

16 

T9 

Ne 

Na 

Mg 

Al 

Si 

P 

s 

Cl 

3 

19.9 

23-05 

24.1 

27 

28.4 

31 

32.06 

35-45 

Ar 

K 

Ca 

Sc 

Ti 

V 

Cr 

Mn 

Fe  Co  Ni  (Cu) 

4 

38 

39-1 

4O.I 

44-1 

48.1 

51.4 

52.1 

55 

55-9  59  59 

Cu 

Zr 

Ga 

Ge 

As 

Se 

Br 

5 

63-6 

65.4 

70 

72.3 

75 

79 

79-9 

Kr 

Rb 

Sr 

Y 

Zr 

Nb 

Mo 

Ru  Rh  Pd(Ag) 

6 

81.8 

85 

87.6 

89 

90.6 

94 

96 

*  * 

IOI.7  103  106.5 

Ag 

Cd 

In 

Sn 

Sb 

Te 

I 

7 

107.9 

II2.4 

114 

II9 

120 

127 

127 

Xe 

Cs 

Ba 

I* 

Ce 

.    (  ..) 

8 

128 

132.9 

1374 

139 

140 

9 

•  • 

Yb 

*  * 

Ta 

W 

* 

10 

•• 

Au 

Hg 

173 

Tl 

Pb 

183 

Bi 

184 

•  • 

Os  Ir  Pt  (Au) 
191  193  194-9 

ii 

197.2 

200 

204.1 

206.9 

208 

" 

" 

Rd 

Th 

u 

12 

"  * 

224 

" 

232 

*  * 

239 

INDEX. 


Adelard,  of  Bath 48 

Affinity,  definition  of 199 

early  views  of. 195 

influence  of  heat  on 213 

mass . 208 

measurement  of .. 202 

strength  of . »..,.. 197 

Alchemists 43 

Allotropism..: 254 

Analogies  of  ^lenrents 183 

Anaxagoras..... . 14 

Anaximander » 12,  17 

Anaximenes... — . 12 

Ancients,  contributions  of , . . 34 

Arabian  theories 50 

Arguments  of  Zeno 33 

Aristotle 15,  17,  23 

influence  of 49 

Tyndall's  estimate  of 31 

Aristotle's  Natural  History 31 

Artioperissads..... 13 

Ascending  series  of  Gladstone 168 

Asclepiades 6 

Atomic  theory  of  Dalton,  first  publication 99 

inconsistencies no 

in  doubt 151 

origin 90 

reception 106 

Democritus 20 

Epicurus 27 

Kanada 8 

Leucippus 18 


284  INDEX. 

Atom  influencing  atom 252,  254 

Atoms  and  molecules 123,  151 

of  Isidorus 43,     47 

substituted  terms  for in 

vortex 266 

Atomic  weights,  first  tables 100,  102 

Attacks  of  church  on  atomists 69 

Augustine 46 

Avidities  of  acids , 207 

Avogadro's  theory 122 

exceptions  to 132 

Bacon,  Francis 54 

Basis  of  natural  system 177 

Basso 57 

Bayley's  table 181 

Bergman 83 

Bergman's  table 198 

Berthollet , 83,  208 

Berzelian  standard 157 

Berzelius 165 

on  affinity 201 

Boiling-points  of  solutions 150 

Bonds 234 

equality  of. 235 

Boscovich 74 

Boyle,  Robert 64 

Bruno,  Giordano 53 

Cannizzaro 154 

Carlsruhe  congress 154 

Chancourtois,  de 169 

Chemical  change,  velocity  of. 217 

reaction,  heat  of. 204 

Chinese  theories 6 

Church  opposition 43,    69 

Clarke  on  atomic  theory 279 

Clinamen 28 

Combining  volumes 112 

Complexity  of  elements 191 

Confusion  in  theory ... 113 


INDEX.  285 

Congress  of  Carlsruhe 154 

Constant  proportions 82 

Contributions  of  ancients 34 

Coordination  number 231 

Corpuscular  theory 52,     59 

Crookes  on  genesis  of  elements 188 

summary  of  hypotheses 272 

Dalton's  claims 80 

lecture  notes 92 

opposition  to  law  by  volumes 120 

rules 109 

standard 156 

theory 90 

theory,  extension  of 108 

reception  of 106 

Davy,  Sir  Humphrey 106,  149 

Democritus , 19,  20,     21 

Densities,  law  of 121 

Descartes 59 

Details  of  Dalton's  theory 105 

Difficulties  in  Periodic  system 185 

Dionysius,  Alexandrinus 44 

Disturbing  influences  in  affinity 203 

Divisibility  of  matter 265 

Dobereiner's  triads 167 

Dulong  and  Petit 134 

Dumas 155 

Eclipse  and  knowledge 42 

Eleatic  school 19,     20 

Electrochemical  equivalents in 

Electron  hypothesis 270 

Elements,  Chinese 7 

Hindoo 7 

Elements  of  Empedocles 17,     20 

Empedocles 16,     17 

Epicureans 29 

Epicurus 27 

Equivalents in 

Equivalents,  electrochemical 148 


286  INDEX. 

Ether  of  Mendele*eff. 280 

Eudorus 14 

Eusebius 45 

Evolution  theories 190 

Exceptions  to  Avogadro's  theory 132 

Extension  of  Dalton's  theory 108 

Failure  of  Greeks 32 

Faraday 149 

Fischer 86 

Fischer's  table 87 

Flawitzky  on  valence 244 

Frankland  and  valence 226 

Freezing-points  of  solutions 150 

Galileo .' 58 

Gaseous  molecules 117,  131 

Genesis  of  elements 187 

Gladisch 10 

Gladstone's  ascending  series 168 

Gladstone  on  mass  action 211 

Gorlaeus 57 

Graham 226 

Graphic  representation  of  Periodic  system 178 

Greeks  as  observers 30 

Greeks,  failures  of 32 

Greek  theories 10 

Guldberg  and  Waage 215 

Harden  and  Roscoe 104 

Heat,  influence  on  affinity 213 

Heat  of  chemical  reactions 204 

neutralization 206 

Helmholtz'  hypothesis 266 

Heracleides 23 

Heraclitus  16 

Hero 6 

Higgins 89 

Hindoo  elements 7 

theories 7 

Hinrichs 172 


INDEX.  287 

Hobbes,  Thomas 62 

Hoefer 42 

Homeomorphism 147 

Homologous  series 169 

Homoeomerics 14 

Hooke's  vibration  theory 66 

Huxley 41,  42,     75 

Huygen's  67 

Hydrogen,  position  of 186 

Inception  of  atomic  theory 103 

Indestructibility  of  matter 8,  35,     48 

Influence  of  atom  upon  atom 252,  254 

position 256,  259 

Interproportionality 85 

Intramolecular  work 129 

Ionic  school n 

Ions,  theory  of. 214 

Isomerism 255 

Isomorphism,  law  of 143 

Kanada,  atomic  theory  of 8 

Kekule*  on  valence 245 

of  carbon 229 

Keppler 41 

Kinetic  equilibrium  in  valence 247 

theory 213,  260 

Knorr  on  valence 244 

Kopp's  hypothesis 142 

Lactantius 45 

Lasalle 10 

Lavoisier 81 

Leibnitz 68,    69 

Leucippus 18 

Liebig 41,  225 

Links 234 

Lodge  on  modern  theories 277 

Lossen  on  valence 242 

Lubin 54 

Lucretius 27 


288  INDEX. 

Malaguti 21 1 

Marguerite  and  Tissier 209 

Marignac 166 

Mass  action 208 

Mathematical  derivation  of  theory 9 

views 51 

Mendeldeff's  tables 174,  175,  176,  282 

Menu,  Institutes  of 7 

Methods  of  research 32 

Meyer's,  Lothar,  tables 172,  181 

Meyer,  Oscar 263 

Meyer,  Victor,  on  valence 243 

Mitscherlich 143,  145 

Molecular  affinity 206 

attraction 219 

combination 230 

Molecules 123,  151,  251 

Molecules,  gaseous 117,  131 

properties  of. 262 

Motion  of  atoms 18,  20,  21,  28,     35 

Multiple  proportions 87 

Nascent  state 126,  253 

Nature  of  elements 190 

Neutralization,  heat  of. 206 

Newland's  law  of  octaves 170 

table 171 

Newton,  Sir  Isaac 70 

Nitrogen's  change  of  valence 232 

Numerical  regularities 166 

Octaves,  law  of. 170 

Opposition  of  the  church 43,     69 

to  law  of  volumes 119 

Ostwald's  viewsof  valence 241 

Ozone 127 

Paracelsus 41 

Parmenides 17 

Periodicity  of  properties 178,  183 

Peripatetics 23,     29 

Persistence  of  elements 51 


INDEX.  289 

Petit  and  Dulong 134 

Philolaus 22 

Plato 22 

Polybasic  acids 225 

Position  of  atoms 256 

in  space 259 

Pressures,  law  of. II7 

Protyle 164,  188 

Proust 82,     88 

Prout's  hypothesis 164 

Pythagorean  school *3 

Pythagoras T3 

Rankine's  hypothesis 265 

Raoult •  ISO 

Reception  of  Dalton's  theory 106 

Regnault J38 

Reversible  reactions 216 

Richards  on  valence 246 

Richter 84 

Richter's  table 85 

Riecke  on  valence 243 

Roscoe  and  Harden 104 

Rose 209,  210 

Rutherford's  hypothesis 278 

Saturated  and  unsaturated 233 

Self-saturation 236 

Sennert,  Daniel 55 

Shoo  King 6 

Solutions,  boiling-points  of. 150 

freezing-points  of. 15° 

vapor  densities  of. 15° 

Specific  heats,  difficulties  in  law 136,  138 

exceptions  to  law •  14° 

failures  in  law • 141 

law  of. 134 

Standard  for  atomic  weights 156 

Substituted  terms  for  atoms m>  I52 

Telluric  screw l&9 

Temperatures,  law  of. JI8 


INDEX. 

Terms  substituted  for  atoms in 

Thales  of  Miletus 12 

Thermochemical  deductions 205 

Thomson 91 

Thomson's  theory 266 

Torricelli 61 

Triads  of  Dobereiner 167 

Turner 165 

Vacuum  of  Torricelli 61 

Valence  as  kinetic  equilibrium 247 

changed  by  chemical  action 239 

electricity 239 

heat 238 

light 237 

Valence,  changes  in 236 

definition  of 223 

development  of  idea 225 

relative 227 

variable 228 

Van  Helmont 52 

van't  Hoff's  hypothesis 240 

Vapor  pressures  of  solutions 150 

Velocity  of  chemical  change 217 

Venableon  valence 246 

Venable's  table 182 

Vibration  theory  of  Hookes 66 

Volumes,  law  of 119 

Vortex  atoms 266 

properties  of 268 

theory  applied  to  affinity 269 

valence 269 

Waage  and  Guldberg 215 

Wenzel 83 

Werner's  hypothesis 231 

William  of  Couches 48 

Wislicenus  on  valence 243 

Xenophanes n 

Zenoof  Blea 9.     J9 

arguments  of 33 


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THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


RC.Q  D  L.L/ 


NOV    51962 


JHL 


LD  21-100m-8,'34 


YC  21616 


